As a dealer... I am more concerned with a person's character or intent, than the number of guns they buy in a single visit.
A person with criminal intent is far more dangerous with one misused firearm, than an upstanding citizen walking out the door with 3 AR15's.
The BATF has no way to track, regulate or speculate on a persons character or intent... so the next best thing is to regulate the number allowed to buy, or track multiple sales. Gun traffickers need to buy in larger numbers to satisfy cross-border demand. The average person isn't buying multiples... but will undoubtedly get caught up in the regulation. I'm not saying I support the idea, only that I understand it from my (FFL) point of view.
If ALL FFL's had the ability to know what a buyers intent was... we wouldn't have this problem... but unfortunately, crooked individuals don't always set-off warning flags. Some are more obvious than others... like the two spanish guys that came into my shop looking for "see-in-the-dark" glasses a few weeks back. A little innocent questioning on my part... uncovered the true intent. "I want to take them back to Mehico with me"
Needless to say, I wasn't interested in his business or his money.