Sharing this from my Oak Tree Press publisher, Billie Johnson. I think there is much confusion among both readers and authors concerning the sales rank of books.
The Amazon SALES RANK number is not a citation of the number of books sold. The SALES RANK number only tells the title’s rank in relation to all the other titles on Amazon, and a book sale is not the only factor in the calculation. 

Amazon shipment
Amazon shipment (Photo credit: enno)

 

Visitors to your detail page, and how the visitor got to your page (that is, did they query at the main page, or did they click-through from another title’s page?), reviews, and numerous other factors play into Amazon’s highly secret algorithm that computes the Sales Rank. The computation is constant and changes as often as hourly. 

A dramatic shift in your sales rank MIGHT mean there was a sale or sales, but not necessarily. 

It is possible to employ a ‘bestseller campaign’ and drive many visitors to your page in a narrow time span and push the Sales Rank number into the Top 100 or some other bestseller list on Amazon and NOT SELL ONE BOOK!

Sales of used copies are counted for rank also, but sales via Amazon   Marketplace are not added to the Sales Report on our publisher’s dashboard. The reason for this is simple: neither author nor publishers are compensated   for the sale of a used copy.

And to add to complexities, Amazon changes things often, so just when you think you have it all psyched out—zap.

Over the sixteen years Oak Tree Press has been in the biz, I have learned to stop worrying about Amazon’s wizardry and be grateful they provide the most level playing field (IMO) of any bookselling venue and we almost never get returns.

Billie Johnson