Businesses that sell supplements should create trusted relationships with their suppliers to not break the rules and to limit their own liability.
The only reason you'd need to go after Amazon is if they're selling products dropshipped from another country, which they could avoid by simply requiring sellers of products meant for human consumption to have a domestic presence. They wouldn't even have to ship from here, just have somebody here who gets arrested if they break the law.
From what I can tell, Amazon does not present itself as a farmer's or flea market, yet it tries to limit its liability by pretending it does. To the average consumer things purchased from it come "from amazon", as compared to ebay which makes it much more obvious you are purchasing from a particular person or shop.
Amazon really tries to have its cake and eat it too here, and it sort of blows my mind that consumer product safety regulators haven't clamped down on this.
It's really one thing to have basically anything available on ebay/aliexpress/others (consumer trust is much lower) and another to be a huge retailer (just like walmart) and yet to be able to sell whatever unsafe stuff you want (unlike walmart).
Amazon will send you another one, possibly from a different seller. That's... weird. It's like they inverted the model where the wholesaler sells to the retailer who sells to the customer. Instead the retailer sells to the customer through the wholesaler, combining different kinds of products into a warehouse instead of splitting up a warehouse with large quantities of the same product into smaller retailers.
But you still want to put things like this on the "retailer" because they're the one who knows anything about the product. It's the same reason you don't put it on eBay or UPS or a self-storage place. And the same reason you don't need to -- you want to shut down or punish the retailer selling the fraudulent product, not all the other retailers or the surrounding infrastructure providers.
Walmart is not a 'marketplace', yet they could easily give you a replacement product from a different 'wholesaler', just like Amazon can give you a replacement from a different 'retailer'. There is no material difference here. Amazon owns everything about the interaction, they have just creatively outsourced creating product listings and a bunch of product risk.
How can they just create a new account to sell supplements without thorough testing, vetting, control processes, etc. by Amazon, the business actually selling the stuff? If Amazon doesn't have such controls in place to stop people from "just creating new accounts" then hold Amazon liable.
Because they, not Amazon, are the business actually selling the stuff. Amazon is a payment processor and a warehouse provider.
You go after the person who knew they were breaking the law, not their landlord or their bank or the dealership where they bought their car by accusing them of not thoroughly investigating their customers. Criminal investigations are the role of law enforcement, not private businesses.
That is legally questionable. When Amazon was losing on that issue in Pennsylvania higher courts, they settled to avoid having an on the record decision that Amazon was liable.[1]
[1] https://www.villanovalawreview.com/post/890-oberdorf-v-amazo...