It used to be that you could go to Amazon and see how many books were left in stock. I always thought this was a ploy to get the consumer to buy right away instead of waiting but apparently its a bit of information that only writers used to evaluate sales. Amazon has recently stopped providing the information on the site. The “number of copies left in stock” information was actually only the number from the fulfillment center and not the entire warehouse so the data did not provide an accurate picture of total inventory.
Authors have very little knowledge about how many of their books are sold, particularly if the author is fortunate enough to be sold at non traditional retailers such as drugstores, grocers and Wal-Mart because those figures aren’t tracked by Bookscan who keeps track of point of sale data. Amazon provided an author with some idea about the number of books sold from a distributor or wholesaler.
In print, as well as in e publishing, the author’s understanding of number of reported copies sold depends in large part on the honesty of the publisher.