And those sellers are just paying for ambiguous marketing services and the fraudulent reviews just happen to get written by someone in another country, who received a small sum in Monero every now and then for some reason.

Where does the buck stop? Amazon directly benefits from having rosy reviews of The products sold on its platform and they don’t do enough to keep their sellers honest.

Amazon is criminally negligent at least, and I'd say enabling the deception in practice. They do scummy things like censoring (rejecting) legitimate negative reviews and letting sellers reassign positive reviews from a product to a completely different one.