I'm wondering more and more why Amazon hosts third-party sellers at all. They dilute the Amazon brand to a tremendous degree. It used to be that buying from Amazon meant a certain level of quality and service. Now there are really two Amazons: the old one, and a new one that's basically a shitty version of eBay. I have to go out of my way to avoid these crappy third-party sellers when I'm searching for stuff on Amazo…
I'd argue that Newegg was more hurt by this than Amazon, because customers were trusting Newegg to deliver a consistent experience at the expense of fewer categories of inventory, while Amazon's offerings were already diluted by a smattering of tiered experiences and bundling with tangentially-related services, of which the Prime/not Prime distinction became increasingly more intrusive.
Though many people use Amazon to find quirky, oddball, one-off products, it does tend to work best when used as an online-only warehouse club -- for repurchasing the same mainstream product over and over; with the added benefit that then you largely avoid having to wade through the filters of 'Sold by Amazon' and 'Eligible for Amazon Prime'. And since they pushed Prime on so many people, they can branch out into eBay's territory with little worries about the health of their customer-facing bottom line.