If it's any consolation, I think many of us who have been selling unique, vintage, OOAK, etc. items for many years are really suffering this year.
As for the reason, I think you're largely right that eBay is no longer presenting itself as a site to find such items. As a result, I think a lot of those buyers have gone elsewhere. The younger, mobile-dependent crowd that eBay is now focused on wants new, shiny, mass-produced gadgetry and fashion of the latest trends.
I also have my own personal theories about the political situation in the U.S. having affected buying habits in my niche market since Trump was elected. There was an almost overnight drop-off in my sales after that point that has never recovered to pre-2016 levels. And I've sold primarily to the U.S. over the more than 10 years I've been selling on eBay.
Of course, a lower sales volume also means not only higher costs through loss of TRS, but also probably lower visibility on .com (where I mainly sell), meaning lower sales, etc. etc. etc. It's not a pretty picture for many of us at the moment.
