Inventory/Purchase Tab - Right Clicking on an item in the right pane allows you to sell more of the items than you have.

PCGen 6.00.0RC1
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.