I wanted to take a moment to clarify yesterday's post. Information isn't a problem when selling a gun in all markets. For example, in a lot of rural markets, most sellers know most of the people around, and most of them know to whom they should not sell, and most of them even abide by the rule of thumb of not selling to the one crazed lunatic drunk in town, and have no more than one or two degrees of separation between themselves and everyone else in their market.
Problems arise in larger markets, because it's hard to know the entire market you're serving. Problems also arise when these smaller markets are preyed upon as havens for unscrupulous thugs who take them back to cities to commit (second or third) gun felonies. That is the point that (mostly libertarian-minded) New York has made about Southern States, including Virginia, whose lax regulations have allowed felons to obtain guns (by mostly legal means) without coming subject to a background check.
Having these background checks as mandatory keeps the local Bubbas from entering a "prisoner's dilemma," or negative-sum strategic game where, even though some would prefer to check buyers' backgrounds, they are trapped by the fact that competing against rivals who may not. As a result even the best-intentioned vendors in these localities are caught in a race to the bottom. Another advantage of making these checks mandatory is that it mandates federal, state, and local governments to show better diligence in keeping "do not sell guns to this man" lists up-to-date, and therefore lowering the monitoring and information-gathering costs of those honest and well-intentioned distributors.