thieves doing this could receive some sort of penalty. Perhaps each counterfeit they sell should mean a lower buying price for any goods they sell, and a lower selling price for any goods bought.

Therefore if someone is offering you dragonslayer for 1 gold, you know it comes from a very suspect source, hence the cheap price. Yet it could always be genuine. Caveat Emptor. (That probably doesn't mean what I hope it means, but at least it isn't Cave Canem.)