I'd say that most of their listings are dominated by independent sellers; you can't even tell if Amazon is offering an object without digging into the "see more sellers".
For example, find a coffee grinder sold by Amazon (note, not Amazon Warehouse Deals)
Amazon keeps indy sellers around so they can use shopping data to identify trends on high-margin, easy-to-produce good that can be readily copied and offered by Amazon for much cheaper.
IMHO.
Plus if you sell on Amazon I bet you buy more too.
Third party sellers are a a cash cow - they take all the risks and Amazon is just a middleman. They make up half of Amazon's sales. Amazon's ultimate game plan may be to drop their retail business altogether and become just a logistics company.
I guess they use it as a win-win if you want a little bit more cost but fast shipping you use them and they win. if you want slower shipping you use a third party and they collect most of the profit and they win.