Stressfire is talking about your Spidey sense. If they guy (or gal) comes across like a goof-ball, well that's your Spidey sense talking to you. I've turned people down before in making the purchase of one of my guns. Something just didn't set well with me on how they talked and such. Asking someone to see their DL is a great way to check their reaction and body language. It's nothing. If they start squirming, or not, you have your answer. Nobody's that hard up that they have to sell this gun to that guy or tragedy.
This all comes back to the ol if it looks like S, and it smells like S...then it's probably S. You can't get mad at a guy for askin for ID on a sale like that, it would make you look suspect.