I'm not sure if it holds for all items, but for the collectibles I buy, most of the sellers are actually companies, which are better served by their own ecommer

I'm not sure if it holds for all items, but for the collectibles I buy, most of the sellers are actually companies, which are better served by their own ecommerce platform, but use eBay due to the discoverability / traffic. There are thousands upon thousands of small stores - I believe the alternative to eBay should be a vertical search engine that aggregates all those stores. Maybe a homogeneus e-commerce platform (with some sort of discoverability for their items) + vertical search engines. For the individual "garage sale" sellers, something like Craigslist or other classifieds, that just has a listing fee and doesn't handle payment & shipping.