I believe that you are mistaken. If the gun was found to be stolen, you would be required to return it. I have never seen anyone prosecuted for mere possession of a gun that turned out to have been stolen, when they otherwise bought it legally. How could you know it was stolen or not?
I would have not had any problems with buying guns on the free market.
Many of the pre-1968 rifles and shotguns do not even have serial numbers.