It's a way of showing how much one has contributed in the forums, thus letting people know their level of understanding of the publishing process here at lulu.
Your Sales rank is how many books you have sold in comparison to all other books sold on Lulu.com. The number takes both self and third party purchases into consideration.
Gosh, sorry! I completed misread and misunderstood the question! My apologies.
I think Glen, the Lulu Moderator, is wrong with the sales rank. According to previous information provided by Lulu,
the sales rank does not depend on the numbers of copies sold but the overall selling sum generated by sales
on the lulu.com website for a title. A hundred dollar book sold ten times weighs therefore more heavily than a
20 Dollar book sold twenty times.