Take a Wizard Oil that you've used four charges of already and want to sell it at full price, or do you want to make sure you don't get scammed?
Well then, advertise in trade using a wizard oil, or other chargeable item that still has it's full five charges.
when someone is willing to buy this is what you do: In the trade window only put up the Wizard Oil with one charge on it, since it only shows that it's Wizard Oil and it does not show the number of charges in trade.
You have sold a wizard oil charged the buyer for a wizard oil with 5 charges left, and you've sold him/her one with only 1 charge left =D
PS: This can be used with other items as well.