I think the eBay marketplace has changed a little, and it may not be that there are fewer buyers, I think maybe fewer buyers are looking for pre-owned, vintage and collectible items. High demand items are selling - even in the collectible area. But...there has been a gradual shift - I think, in the type of buyers that eBay is attracting.
With few exceptions, I have experienced used, vintage an collectible items on eBay simply not selling well. Used books in nice condition do decent. LP's do decent. Pre-owned jewelry actually does well. But... a lot of used items have near the demand on eBay that they once did. There are a ton of alternatives for buying these items that have popped up in addition, people are able to buy from social media tools as well.
So, you have to be cognoscente that eBay is simply one outlet now for these types of items and it is not the inventory they are really pushing. Add into that equation that their search function leaves a ton to be desired and well, more sellers are having a very hard time of being found.
I think the GTC is one way eBay is looking to change how traffic comes into the site. This should see some benefit, if sellers are willing to adapt, in people not presently active on eBay finding the items they are looking for on eBay. Those stable links will improve indexing and help search results and google shopping. The issue beyond that for sellers, then becomes the cost of selling if you are trying to sell more inventory than your store size allows. The per listing costs will just eat a lot of sellers profits right up - unless sales actually blossom as a result of the changes.
It will be interesting to see how this works out. I understand both ends of the spectrum in this as both a seller and someone who owned a internet development marketing company.
Cheers