All authors can see an info like this (in their portfolios):
Purchases: 84 (21.00 p/month)
Item sales: $3,449.20 ($862.30 p/month)
The numbers in brackets come from a simple dividing of total sum by number of months. However the sales of most items on the market are not linear and not permanent. The sales have a hyperbolic nature, not linear. Such calculations completely meaningless, and should be removed or replaced with something relevant e.g. estimated sales in current month.