I wonder if a legal notice in your local paper would protect you.
If a person doesn't care enough to get a title, **** em. Been in a mess with that and the DMV before. DMV gave the guy who bought my old car new plates in my name, then bitched at me for not taking the plates. But I did. Screw...
Do you mean the barrel or bolt carrier group? Parkerizing is the standard finish. Nitride is supposed to be harder and slicker. The AR upper receiver itself is aluminum and usually anodized.