boofus says "I'm amazed some of those jokers can stay in business"
Maybe the jokers stay in business (and out of prison) by not taking unnecessary chances with sales to people they aren't familiar with, who have told them outright they are planning to do something the FFL thinks may be illegal. In the dealer's defense, if he didn't know you, how did he know if a machinegun in your bag was legal or not?
I said above I quit doing gun shows over 5 years ago, and one of the reasons was folks asking things like "what would it take to make this AR fully automatic?" (I don't know and don't want to know), or "I'd buy this shotgun if you would cut the barrel off shorter" (yeah, right), or "I want to buy AK's in large quantity, what can you do if I buy 200?" (I can ask you to move along). These aren't made up questions; I had all of them asked of me at gun shows, and many other things similar.
Many folks don't understand just how scary it can be for FFL holders when customers start to ask (or worse, demand) that the dealer do something that may not be legal. At a gun show, you never know who's across the table. It could be a gun enthusiast, it could be a nut, or it could be a BATFE agent just waiting for the wrong answer to make your life miserable.
Most dealers don't get rich at this, and when it comes down to a questionable sale with a $25 profit, or waiting for someone not so scary, the sale can wait for another day.