Grandpa kept telling his grandson that he sure wished he never had sold his Marlin rifle...
Uh oh. This kind of changes my answer. If there is a chance the grandson wants to "return" the rifle to the grandfather before he dies, I would sell him the rifle. As Feralmeril says, selling it to make grandson and grandfather feel better before the old man passes on probably won't change your life - financially.
If it is something else, then I would stick with my earlier post.