For years it was rumored that several authors and publicists had posted flattering reviews of their own books on Amazon, anonymously. This dishonest tactic of writing shill reviews, sometimes called "astro-turfing," depends on contrived reviews to simulate a grassroots movement for a book on Amazon.
Then in 2004, a computer glitch revealed it was true--the real names of the authors were displayed, earning them a lifetime of embarrassment. One was John Rechy, author of the bestselling novel City of Night. The ironic thing was that Rechy was a successful writer whose honors included a PEN-USA West lifetime achievement award. He wasn't famous, but he didn't need shill book reviews either. But that computer glitch made him much better known, though probably not in the way he'd hoped.
One medical doctor who has a book for sale on Amazon has submitted hundreds of reviews of other books, which serve primarily to point attention to his own book. Apparently the doctor isn't concerned that his reputation as an author has been tarnished, as he's continued the activity.
In response to years of controversy about abuse of its review system, in 2006 Amazon began requiring that reviewers have an account with a registered credit card before reviews can be submitted. The safeguard prevents individuals from using multiple accounts to submit phony reviews. However, customers aren't required to purchase a copy of a book from Amazon before reviewing it.