Imagine if you were standing in front of a narcotics officer on the street, and you say to your friend "Hey, I have some Cane-Coke available. wink. Want to buy it?" He's standing right there, and doesn't bat an eye.

That's Amazon. They care about following laws, regulations, etc. exactly enough to have plausible deniability and no further. Oh gee, Sarge, that guy was speaking in code and I had no idea he was selling drugs.

If they cared, they'd ban sellers immediately for evading a filter, and raise barriers to entry until it was painful to start a new account. Like requiring every seller to have a US entity with a real business license and an identity-verified named agent, and ban the agent and anyone else they represent for violations. This is just one quick idea but by no means the only way. But you can bet Amazon would never even try to police their marketplace better because they'd rather just skim their cut of both legitimate and fraudulent or illegal activity.