If you know for some reason that the person can't own a firearm then you can't sell it. If you believe the person is legal to own a firearm then you should have no problems.
You can ask if they are a felon and if they can own the gun but you are not required to check. I would suggest that if have any doubts then don't sell it.
I would say that if you do sell it, at least give a signed receipt and keep a copy for yourself. This way you can prove you sold the gun if it turns up in a crime. You are not required to do this though.
[url]www.ar15.com/forums/topic.html?id=64420[/url] It looks like he is just being careful.