Wonder what mom and pop paid for it? Is it hot, or did they simply think it was junk? Even if it didn't work, it's worth at least twice that much.
I know a business deal is a business deal, however, because they aren't gun people and may have unwittingly sold me something worth much more, from an ethical standpoint, I would have went back to them and let them know of their mistake and simple repair and would have offered more. If they declined, then I would sleep better. I know I'll probably get flamed, but that's the way I was raised.