Commerce product order/sort options
When you call craft.commerce.products you're asking Craft for an Commerce_ProductModel. 

So you could sort by:


title (alpha)
defaultPrice
type
promotable (sale products on top)
postDate (newest products)
.. etc.


To sort by sales (popularity), you might consider something like BonFire's Popular Products plugin which keeps track of how many purchases were made and how many times the product was viewed, very similar to Ben Croker's EntryCount plugin.