Of course I understand why this is wrong and unethical, so I tried to report it to Amazon.
There is no way to report this to Amazon. They have no facility for reporting a product that someone tried to pay you to review positively.
The only thing you can do is contact customer service who as far as I can tell do nothing with the report.
All they need to do is add a category to their customer service section to report fraud. It blows my mind that they do not do this. But not really. They make more money by allowing this behavior, both from the fake buyer, as well as the other customers who are duped into buying highly reviewed items.