This is also one of the tricks Amazon uses to keep pricing low.
My understanding is that this is most of Costco's profit model too: they take merchandise on consignment for say 60 days, sell it all at cost within a week, and then invest the float.
Incidentally, I know a wholesaler [1] in London which sells at almost cost price. It makes most of its profits from sitting on the cash it takes for 60 days, before paying it to suppliers.
[1] http://www.dhamecha.com/