I'm pretty sure what the companies selling reviews are doing is unlawful in the EU under the Unfair Commercial Practices Directive - the reviews would need to state they were done for a consideration and by a business in order to meet with legal requirements from what I understand.

There's a guide to the directive based on UK law at http://www.out-law.com/page-9050:

>'A practice is unfair if it fails to meet the standard of "professional diligence" (the standard of skill and care that would reasonably be expected of a trader in its field of activity) and it materially impairs an average consumer's ability to make an informed decision, causing him to make a decision he would not otherwise have made.' (Out-Law.com, ibid) //

If you wanted to get creative, posting reviews on Amazon counter to their Terms of Service could run afoul of the CFAA.

I'd claim it is illegal, and fraud. If a seller buys positive reviews, then anyone buying the item has been led to believe that the reviews are independent and honest reviews. The seller didn't inform the buyer of information that the seller knew, and the seller materially benefited from not informing the seller, ergo fraud. It's a crime as old as the hills.