Chris Anderson, in the process of writing The Long Tail, is highly motivated to learn exactly what Amazon.com’s sales ranking numbers mean. Today’s post on the subject is fascinating:
Amazon still isn’t releasing the hard numbers, but we do at least have a bit more experience at reverse-engineering them. A number of other academics have taken a stab at it since then, as well as some independent experts.
I asked one of them, Morris Rosenthal of Foner Books, to extend the analysis he’s done over the past two years on correlating Amazon sales rank to absolute sales and apply that approach to the Long Tail. Fortunately, he took to the project with a vengeance and has built the beginning of what may be the best analytical framework for estimating Amazon’s sales yet.
If you’re interested in how someone reverse-engineers the workings of Amazon, read Rosenthal’s excerpt at the end of the post. It’s fascinating.
Ed Bott 
Unscientifically but intuitively I consider Amazon sales rank to be a fair representation of the sales number for most mainstream products. For example if you are comparing MP3 player sales numbers the Amazon sales rank shows iPod ranks in top 100 while all others like iRiver or Sony MP3 players rank poorly. Then you compare the sales numbers reported in respective earnings reports that correlates. For a simple amazon sales rank tool see this link.