I don’t know about where you live, but in my part of the world, a private seller is allowed to rent a table for the sale of items from his/her private collection. That’s an entirely private sale, no background check, no paperwork, no forms, cash-only if he wants, on the floor of the show. See this article for a description of a show organized by the company that holds regular shows in the Midwest, including my hometown.
I’ve even seen a vendor who had tables “over here” with guns requiring background checks and a table “over there” filled with guns allegedly from a private collection; the vendor openly advertised that the guns “over there” on the adjacent table were being sold without background checks. He himself held an FFL, and sales from his stock required a background check, but he was facilitating dispersal of a private collection. Show management was aware of what he was doing, and had no problem with it. That was years ago, and he still has an FFL, so apparently the feds didn’t have a problem either (or never found out).
Parking lot sales are not illegal in the majority of states, including mine.
The person purchasing commits a single felony under 18 USC 922(a)(3). (a)(5) is indeed focused on the seller, and requires “knowing” your purchaser is not a resident; “don’t ask, don’t tell” isn’t just for the military, and no law requires a private seller to make inquiry. If your possession of the gun is a felony in your home state (felon in possession, e.g.), the same holds true for your acquiring it at home; there’s no need to go to another state. California is fairly unusual in having a lot of restrictions surrounding the import of guns, and most states don’t have such rules.
In other words, most people going to another state, buying a gun, and bringing it home are committing a single felony.
The question, however, is whether having a gun all nice and handy makes somebody more inclined to use it even when its use is not appropriate (the old guy who pulls out his piece to settle an argument over a fender-bender, e.g., as happened awhile back in my neck of the woods). The answer to that question requires knowing the crime rate for CCW holders versus other people of similar age, etc., who don’t carry, and we don’t have those kinds of statistics.