Network effects. Sellers use eBay because buyers use eBay because sellers use eBay. eBay dominate the long tail of e-commerce and will do for the foreseeable future.
You can challenge eBay locally, as shown by Craigslist and Gumtree. You can challenge eBay in a specific niche, as shown by Etsy. Facebook tried to take on eBay directly with Marketplace, with limited success. Amazon Marketplace competes fairly directly with eBay, but at a significant cost to Amazon's reputation - it has flooded their platform with counterfeits and fake reviews.
From a buyer's perspective, the network effects are strongest when you're shopping for one-of-a-kind items. If you're a coin collector looking for a 1896 half-crown in grade 62 toned, you have to be on the largest marketplace to have the best chance of getting one.
If you're seeking commodity items, the network effect is far weaker When I need a cheap USB 2.0 hub, why should I buy it from eBay when it will likely be a buck or two cheaper from a DealExreme/Banggood/LightInTheBox style direct-from-China shop? The selection will be adequate even if there's only one seller.
Similarly, as a seller, a combined marketplace makes sense when you have narrow inventory drawing low traffic. You have two widgets, and they'll sell faster being listed inside eBay's assortment of 500, than by trying ti SEO your two product pages. A lot of the classic eBay product lines were stuff where there were few "category killer" web shops with wide inventory and workable sales flow. (Think of the baseball card dealers where they publish paper price lists)
I wonder if in 10 years we'll see an eBay much more like that from 10 years ago. The cheap commodity items will eventually move to sellers' own sites where they aren't giving up a major cut of the purchase price, but more differentiated individual items will stick around.
Yes, as a customer, I think Amazon has been accumulating negative aspects of Ebay marketplaces faster than Ebay has been acquiring positive aspects of Amazon. That said, I have recently found myself buying some products on Ebay (w/o going thru the auction rigamarole) because they're priced better than Amazon .
You can challenge eBay locally, as shown by Craigslist and Gumtree. You can challenge eBay in a specific niche, as shown by Etsy. Facebook tried to take on eBay directly with Marketplace, with limited success. Amazon Marketplace competes fairly directly with eBay, but at a significant cost to Amazon's reputation - it has flooded their platform with counterfeits and fake reviews.