It's important to seperate Amazon the market place and Amazon the merchant here. Amazon is a market place where you can find all types of merchants. In this role, it's not so much Amazons issue what these merchants sell.

It's similar to how there are all types of websites running on AWS. Amazon is not responsible for their content.

Amazon is also a merchant on their own market place. I would expect Amazon and other big name merchants to usually sell you genuine products.