By this rationale, any used gun bought anywhere could be potentially not legit. Think of it like this:
You buy a used pistol. You arent super familiar with handguns, but its a good price at your local gun shop.
You bring it home, shoot it a little, clean it, put it away.
You have no reason to expect the fuzz is coming to lock you up for owning this thing.
According to this thread, you have EVERY reason to expect the fuzz to come take you away.
Why? Because the serial number has been re-stamped? How is that criminality on the part of the new owner?