And once again we see the difference in definition between Federal law and states.
As NavyLT pointed out, as far as the feds are concerned, he's a resident when he decides he wants to live there. But what about state law? There may or may not be one that applies.
As was pointed out, the language of the law may not require you to see proof of residency, just have no reason to suspect the buyer is not a resident. A driver's licence would do that for me. So would a pistol permit, or other kinds of id. While it may not be a legal requirement, the law doesn't prohibit me from being as nasty and suspicious as comes naturally to me.