No, I don't think it's the promotion percentage. I've run tests on that, high percentages haven't made items move. In fact I found that the low percentages sold better, paradoxically. I think buyers have learned to skip the promoted listings, and the real value is in the non-search impressions.
In some categories the high percentages might work. In others, people might just think they do...they might be getting misled because any edits to a listing will tend to cause a brief boost in visibility.
(Paradox: I have also found that if you pick up a stale listing and use "sell similar" to list some other new items like it, it isn't uncommon that the stale listing will suddenly get offers! Try it.)
What I think is that Ebay's internal rating system, whatever it is, is unstable, and easily confused. And when it gets confused, your listings get 'streamlined' away in searches. Even if you have a lot of stuff that people would usually want. Even if your prices are good and your metrics are stellar.
I think that if you sell a lot of stuff in the same category, sell 10 items a day in mostly that category, backfill with 10 new listings in that category, your sales stay at that mark. If you instead list 20 items a day your sales still stay up, maybe get a bit better, but you start building up inventory. At some point you are like us with 1000 or 5000 or 10000 items listed.
But that's when things get dangerous, because for whatever reason, when you grow, the daily sell-rate never quite grows at the same pace as the inventory. It "smells" like there is some sort of inverse-square rule at play, though I doubt it's deliberately programmed. But then you get into problems, because suddenly you have all these legitimately good, attractive items (if you listed them anew, today, they'd sell as quickly as they did, when you listed others before that were like them), but the daily sales rate doesn't turn them over fast enough, and Ebay decides that suddenly, the items you had that were so interesting two years ago when you had less...are just not that interesting, because, hey, we auto-calculated the sell thru rate, Q. E. D.
Basically I think you're doomed, on this platform. You're doomed to grow a whole bunch of inventory, then die. And every time I see a big seller on here with the same sort of experience, I get more certain that this is simply how this platform works, whether by design or by simple bad programming.
There are items I'm quite interested in for my own use, for which the market today is as good or better than it was 10 years ago. But 10 years ago I was discovering sellers when they had 300 items, and watching them grow rapidly, sales growing at the same time, until they had 10000 or more, and with the sales to match. I am not seeing that kind of growth anymore. Sad.
