The integrity of the inside is the main factor. Other than that, you just want them to hold pressure. None of mine have anything more than minor dents.
Any dents around the lip of the top opening or bent lids would disqualify them. Lid seals are touchy enough without dents. Chipped plugs are also a stopper. Side dents don't matter. In fact, any damage around the top is a potential problem.
ok, side dents are ok...................................they guy at the scrap yard said "it even has the lid" like it was a surprise. I wonder what that means?