Contraband

Law enforcement officers are expected to turn in seized contraband including magic items into evidence.

If you keep it and use it, there can be repercussions if caught. This depends entirely on the situation. Some items are harder to use without giving their nature away. Being greedy and never officially finding any contraband might cause suspicion. If you're caught, who are you caught by?

''As you can see, no hard rules here. Only story opportunities :-)''