Buy from someone who is not a FFL dealer, then no 4473, no crime. Are there collectors at the 1500, who buy and sell for personal use? I think if he were not legally able to purchase for himself, and you bought it for him, then that's a crime, JMHO
I might be wrong, but I didn't think you could sell from a table or booth at the 1500 unless you were an FFL. I know people are allowed to bring guns for private sales though.