The big difference between Amazon and other marketplaces is that Amazon have done their best to not make it look or act like a marketplace. Like, if you look at a listing on eBay there's a prominent seller information pane telling you which user you're buying from, their feedback history, etc. On Amazon there's just a single unobtrusive "sold by" line which blends into the rest of the page. Not only that, if you or someone else comes back to the same listing a few hours later the product could be coming from a completely different seller - Amazon decides who they want to direct your purchase to based on secret algorithmic factors.