Considering how much of Amazon has become just merchants who buy on Alibaba and sell on Amazon, or even outright Alibaba sellers, I’m shocked that it hasn’t had any blowback.
Most likely Ebay. Amazon is built around a bait and switch tactic. If you buy anything directly from Amazon, the risk of a scam is minimal, this is the bait. However, the switch tactic is that they made it increasingly hard to order directly from Amazon by co-mingling and progressively decreasing the UI distinctions between items sold by third party sellers and Amazon. When it turns out that the article is popular and profitable, Amazon will introduce a "Amazon Basics" version of the product, where the product is branded in such a way, that it is obvious that only Amazon can sell it.
The core distinction between Ebay and Amazon is that on Ebay each shop+product combination is its own listing and the seller reputation is put up front, with the buy button below the seller information. Meanwhile on Amazon there are product listings, with multiple sellers providing the same product and the seller name being put below the buy button. There is no obvious indication that you are buying from a third party seller on Amazon, whereas on Ebay you can only buy from third parties, so expectations adjust accordingly.
I suspect many small players have simply stopped playing entirely. Most alternative selling platforms are a shit-show too so implies you are big enough to run a chunky platform yourself it is difficult to compete with the low quality low margin players.
Amazon is now for the “we technically have QA but honestly don’t give a shit” sellers. Not quite alibaba but getting there.