Exactly! But there are a lot of "dealers" who masquerade as "private citizens" at shows. Additionally, about three or four years ago I attended an estate auction with a buddy. He purchased a .45 colt 1911, a S&W .357, and a Remington Model 70 at the event. Nothing was required except his personal checkbook for the firearms. I know that he was okay, but what about the next guy off the street?
It's my understanding they're breaking the law. In that case the issue is enforcement, not another law. I'm not a frequent show attendee, but I'm told that most shows are pretty strict on dealers due to potential liability to the sponsors, but that might be a regional thing. An estate auction, if it's conducted by a commercial company, in my mind they should be held to the same standard as dealers. If it's an individual, like a house sale, you're right, they'd get a pass. I guess the real question is whether true private sales are a risk, my guess they're not or we'd be seeing statistics cited. Not the typical straw man of gun shows or "private" sales to gang bangers from the trunk of a car.