Sadly, at this point Amazon has the network effect of both consumers and suppliers. They've got the eyeballs, so people want to sell through them, so they've got a diversity of items and people check there to buy first, repeat repeat.

Be that what it may for the market: I think they've been a lot less mercenary than they could be (they could likely jack fees a lot higher now without much fallout), but the potential is certainly there.

Great for low cost optimization, bad for great feature development.

PS: The whole only-being-able-to-price-sort-in-a-single-category is terrible too. How do I know if Thing X is in category Y or Z? It successfully reproduces all the fun of wondering what aisle the grocery store stuck trail mix on.