by Derek Haines on Just Publishing Advice site:
Amazon combats fake book reviews with $50 minimum spend
Book reviews on Amazon over the years have attracted more scams than can possibly be imagined. Most revolve around click farms and amassing lots of Amazon accounts.
By creating hundreds, if not thousands of Amazon accounts, scammers could boost free Kindle ebook downloads, add customer reviews, or manipulate book sales ranking.
There have been many, many articles written about problems associated with Amazon customer reviews in relation to ebook reviews in particular. There has also been manipulation of Kindle ebooks, as Let’s Get Digital explains how Scammers Broke the Kindle Store.
At the same time, Amazon has tried to resolve these problems in a number of ways.