Here is my personal experience with this matter, coming from someone who has been selling for 15+ years.
A HUGE portion of my Ebay inventory has been sitting here, on this site, for 5+ years.
Yes, 5+ years.
I have lowered prices, I have given these items a promotional rate of 10-15%, I have changed keywords, reworded tiles, re-photographed. Yet, I have hundreds of items that have been sitting in my store for 5+ years. That's hundreds of items that no one has wanted in over half a decade.
Meanwhile, I'll sometimes source newer, fresher, more in demand, and more exciting items for really cheap. I often make huge bundles and end up buying out a lot of people who want to liquidate for cheap simply because they don't want the items anymore or they are going out of business.
There have been times when I'd bring in a huge haul of 200-300 brand new items and I'd get all 200-300 items listed within one month, and, about 80% of the entire inventory would be gone within one month. Many times, I'd actually list an item and it would sometimes sell within days, sometimes within hours, and even sometimes within minutes of me listing it. Recently, I listed a brand new item and someone bought it five minutes after I listed it.
Then, I'd notice that if some time passed since my last exciting and fresh haul, things would slow again. I'd be stuck with those same hundreds of items that have been in my store for 5+ years. Nothing would be selling. I'd go hunt for more liquidations, for more thrift store sales, for more garage sales, for more people willing to give me a bunch of stuff for cheap because they're going out of business or view this stuff as junk (despite it selling very well online)
So, I'd sulk for a few days about how it's Ebay's fault, and sales are slow, etc. etc. Then, I'd sort of snap out of it and I'd go on a mission to source more cheap items through my contacts that I've made and all of the aforementioned methods that I use for sourcing items. And, ta-da, I'd come across another great haul of several hundred items that are newer, fresher, and more easy to sell than a lot of the hard to sell stuff sitting in my inventory for over half a decade. I'd bring these new hauls in, list them, and things would go flying off the shelves.
Then, once they flew off the shelves and I ended up selling hundreds of items within one month, I'd be back to being left with hundreds of items that have been on my store for 5+ years. Once again, I'd sulk about slow sales, but would then go out to source more items. Rinse and repeat.
Is it more likely that I have literally hundreds of items in my store that have been in there for 5+ years because Ebay is doing something to make people not buy my items (despite me putting a 10% promotional rate on them, despite me running sales on them, despite me re-arranging keywords, re-photographing them, posting them on social media, etc.) or... MAYBE, just maybe, the reason that some things sell almost instantly and some things (as I've mentioned) have sat in my store for 5+ years is... simply because no one wants them.
And that's the cold hard truth. Those items that I can't seem to sell no matter what I do? Nobody wants them.
You can talk about conspiracies all you want. They may be true, they may not be true. Maybe there's a middle ground where they're half true - I don't know and I don't want to get into a debate about what I do or do not know to be a 100% absolute and concrete fact. What I do know, however, is that there's no way that hundreds upon hundreds of my items have been being throttled, have had search engine issues, have little page views, and have been limited in visibility for over half a decade simply because it's Ebay's fault. The truth is, no one wants those items that have been in my store for over five years. No one wants them. I have seen it time and time again. I'll come home with a 400 item haul of things that are more in demand and more desirable and it'll sell like hot cakes while I'll still have those several hundred items in my store that no one absolutely wants, despite my best efforts to try to sell them.
The sad reality is that even if Ebay does limit visibility, and even if you aren't in the search engine 100% of the time, and even if the search engine can be wonky sometimes and not put your search result at a good placement... the sad reality is that your items will only sell if someone wants to buy them. And, not only that, but we're also competing with 100s of 1000s (depending on the category) of sellers who are selling the same exact things that we are. Plus, if someone is even slightly higher in the search results than we are, or has a slightly better price, or slightly better photos, or a slightly better description, then they can take a sale away from us that could potentially be ours.
Categories are becoming over-saturated, crowded, and some people get better placement than us in search results -- especially if these people have a lot of buyers who follow their store, for example. People with higher promotional rates also get way better placement than those without a promotional rate or those who have a low promotional rate. Even if these theories about Ebay are true, people still need to realize one thing. People speculate that the search engine is broken, that throttling is a thing (I'm not saying it isn't a thing, Ebay doesn't exactly deny that they never hide our listings), and so many other conspiracy theories about why our sales aren't happening. HOWEVER, even if throttling 100% stopped, and even if the search engine was better, and even if all of these problems went away, people would still be making the same boards about slow sales, no sales, or Ebay doing something to hinder sales.
Why? Because even if all of these problems that people speculate about were 100% fixed, we would still be battling extensive competition, over-saturated categories, Chinese sellers (who sell much cheaper than us), our items simply not being in style anymore, people buying the same items we do sell from other websites, and we'd still be competing with thousands of other sellers who are selling in the same category. It would still be chaos even if every single issue that people are speculating about was fixed.
Would things improve, though? Maybe, yeah. But the main thing is: 1) your items won't sell if people won't buy them in the first place 2) Your items may not sell if you don't have the best price on the entire site 3) your items may not sell if you're not high up enough in the search results
And those are all cold hard facts. Even if you stripped all of the problems that people say Ebay has, we would still be tackling all of the above and it still would not by any means be smooth sailing. It might be smoother sailing, but it still wouldn't be smooth sailing.
I also want to add this:
People need to remember that we're not just competing with thousands of sellers within the categories that we sell in here on Ebay. We are competing with every single large and small vendor on the entire internet, whether they are a small vendor or a huge mainstream marketplace. We are competing with: other online marketplaces, auction sites, storefront websites that sell the same things we do, Craigslist, and now there's Facebook marketplace which wasn't in place before. I have met many local ex-Ebay sellers in my area who swear that their sales tanked when our local Facebook marketplace because so active. On top of that, we are also competing with in-person vendors.
A good way to put it into perspective is like this:
Seller A has an item.
Buyer A, B, and C want this said item.
Buyer A may find this said item that Seller A is selling on a plethora of other online marketplaces completely outside of Ebay.
Buyer B may go out and buy this item in person.
Buyer C wants to buy this item on Ebay itself and no other online or in-person store.
Now, Buyer C has to sift through hundreds or thousands of search results.
The ones with the best price, the best photos, the best descriptions, the best visibility, the highest promotional rates, etc. are all fighting to attract Buyer C to buy their item.
Meanwhile, Buyer A got the same item elsewhere on the internet and Buyer B bought the same item in person.
Now, Buyer C has to sift through Ebay's search results to try to find the best value for their buck.
Buyer C may even change their minds while searching for items on Ebay and decide that they want to get the item elsewhere online from a different marketplace or try to go out and buy it in person like Buyer A and B.
In a metaphoric way, buyers are like fish that we sellers try to reel in, but, they sometimes just get loose and simply swim away.
Hopefully that put things more into perspective. Remember that as Ebay expands and grows with more and more sellers flooding in to sell their items, there are also places elsewhere on the internet on different marketplaces as well that are expanding. There are places in real life that are expanding. We have more sellers now than we did in the past, but also, there are now more other online marketplaces where buyers can get items from. People often say "I remember when Ebay was different and things sold quickly..." I remember those days, too.
1) Ebay had a LOT less sellers on here back then in those days.
2) There were not as many other online marketplaces such as Facebook marketplace as there are now
3) Yes, search engines and algorithms definitely changed. Yes, throttling may be a thing. I'm not discounting it.
4) Remember, things go out of style and out of fashion. Older generations may stop buying a lot of the vintage / collectible stuff that yard sales and thrift stores often have and the newer generations (who were not using Ebay 10-15 years ago) are now using Ebay and shopping for completely different items than what were commonly popular 10-15 years ago.
5) Add in a ton of extra factors that I have not mentioned or listed, and, you have a recipe for why sales might not be as good as they once were. I, of course, can't list every reason or name every algorithm issue that may exist. I can't go into every search engine issue, or every website flaw that I'm sure impacts visibility and our sales, but, I'm pretty sure that we all know by now that this is not a very well oiled machine, so, obviously sales going south and things going wrong from time to time (or being on a steady decline, even) is not out of the realm of possibility, but, this could be for so many different reasons. It could be a 50/50 combination of Ebay's fault vs the seller doing some trivial thing wrong such as being off on pricing, or not having good photos, or a good title, etc.
All in all, I think that there are so many different factors and elements that factor into sales being potentially slow, and, I see a lot of people sitting on the forums and speculating about what Ebay is doing wrong. "Ebay is doing this...", "Ebay is doing that." - OK. Even if Ebay is doing all of these aforementioned things that are wreaking havoc on our sales, we have absolutely no control over it. However, even if Ebay's system is at fault and is causing slow sales for us, I am fairly certain that if that is the case, then it is merely a piece of the puzzle. If the site itself is doing something to hinder our sales, then, we cannot control it. There are other pieces of the puzzle here to look at. If Ebay being wonky is only one piece of the puzzle as to why our sales are low, then we must re-evaluate the pieces of the puzzle that we can control. What can be control? Sourcing better items, changing keywords, taking better photos, writing better descriptions, having sales, adding promotions to items that aren't selling, advertising our Ebay stores on social media platforms, listing new items daily to try to bring in traffic, etc. I can't tell you how many times I'd list a brand new item and the buyer would message me to let me know that they saw my newly listed item, ended up looking through my store, saw some stuff that they liked, and ended up wanting to buy 5 items or so that have been in my store for ages. It happens all the time. I'll list something new every day, and, sometimes whoever buys the newly listed item will scan my store for other cool stuff, and, they'll end up buying an item that has been in my store for 5+ years that was otherwise not selling whatsoever beforehand.
While we may not be able to control everything about our selling experiences here on Ebay, and while we are all probably aware that there are many external factors that may weigh in on why sales are slow, let's not forget all the things that we can control and do have some power over and some say in.
Good luck to you all. 
P.S: Before anyone comes at me with torches and pitchforks, no, I am not an Ebay cheerleader. I am not dismissing the issues Ebay does have. I am not 100% happy with my selling experience nor did I ever say that I was. Sometimes, the lack of sales on slow months really does get to me. I am fairly neutral because I'm not polarized to one side of the spectrum. I see so many polarized posts on the forums. There are some people that are 100% blaming Ebay for slow sales and there are some people who are 100% blaming sellers for the issues. Me? I'm somewhere in the middle, thinking that maybe the blame is 50/50 + blame should be given to all of the external factors that I mentioned that might be affecting things. I'm just trying to counteract whatever selling obstacles are thrown at me by doing things that are actually in my power that I have some form of control over, and, I just sort of wing it and hope for the best.
