Had it happen to a friend of mine. He sold a shotgun, than over the years changed hands a few times and ended up used in a robbery. The cops came looking for him, and he had to explain and try to prove that he sold the the gun years ago.
If you sell, document it. Or use a FFL to transfer.
Do we really want to be on the slippery slope towards confiscation?
Just because you owned a gun once that was later used in crime years later, doesnt mean you are responsible for that crime.
Should we have to document the purchase and sale of all of our personal belongings as a hedge against them turning up later at a crime scene?