I would look at this from a little bit different point of view than some that have posted here.
1. This gun may legally belong to someone else. Consider how you would feel if someone else had possession of your property, regardless of how they received it. You would want them to return it. I think it is a moral duty to return that which has been stolen from someone. I hope I would always follow that duty.
2. If the gun is actually stolen, it is evidence in a criminal matter. If you have knowledge that it is stolen you could be charged with possession of stolen property, and if you do anything to it you could be charged with altering evidence in a criminal case. Both very serious crimes.
3. You may sue the seller in small claims court. In most states it is not that difficult to collect on a judgment if the person you sue has some assets or some income. There is usually a list of what may be attached or garnished in the event that the defendant does not voluntarily pay the judgment. But in some states you will have to remove the case to the district court and pay extra fees in order to collect on the judgment.
And yes, I am an attorney.
