For potions and oils it is easy. A small paper-thin disk of steel is on the bottom of every potion bottle or alchemist bag, etc and if the item leaves the store without being removed by the proprietor, it works in reverse ways. As in that potion of healing your really needed, now a poison, but the only way to know is to drink it. That oil of flame blade, just rusted your sword, etc.
Permanent items are a little harder, but making them cursed items if they are stolen doesn't seem to far fetched.
All of this is contingent upon your characters trying to be good guys. If they are villians, then it can still happen, it just poses an interesting hurdle.