Data Sets loaded: PFCR, Bestiary, APG
I've got a 1st level halfling wizard with one backpack. When I right-click on the backpack in the right pane of the Inventory/Purchase pane I get the drop-down menu. Pulling down to the Selln/Sell 5 option the single backpack is removed but the money available is increased by 10gp. That is 5 times the cost of the backpack.
The sell action is capped behind the scenes at the number owned now.