(this answer is from Myka submitted by emk)

Additionally, if it helps to answer the part of the question about whether Amazon can prove its reviews are from unique users, there's been quite a flap about anonymous reviews and ratings on the site. Anyhoo, an article about Amazon's latest baby step to put a stop to the bogus practice just appeared in The Guardian yesterday: http://www.guardian.co.uk/business/story/0,,1271178,00.html

FWIW, Bookscan is another source we use in the industry to track book sales, but as with other tracking systems, it doesn't necessarily tell the whole story (in our case, it really tells us our and our competitors' sell-through in the major chains, not so much with independent stores). And, as an industry tool, it ain't free or open to the public. Publishers guard real sales figures jealously, so you're *always* going to have a hard time getting a straight, reliable answer about exactly how many units have been moved for any given title. Myka