AP Exclusive: Pentagon gun was from TN police:

Two guns used in high-profile shootings this year at the Pentagon and a Las Vegas courthouse both came from the same unlikely place: the police and court system of Memphis, Tenn.

As the article notes, the Tennessee legislature passed a law requiring law enforcement agencies to sell confiscated guns. Seems to me that if a gun is taken off the street because it was involved in a crime it ought to stay off the street.

The article also says one of the killers, who was prohibited from owning a gun, bought his at a gun show by way of a "private sale" which doesn't require a background check. The new Tennessee law only allows law enforcemnt to sell to a licensed firearm dealer, but this weapon apparently found it's way to a private owner, presumably through legitimate channels. Seems to me "private sales" (which many times aren't) should be banned at gun shows. (Related)

Beyond that, maybe any "private sale" should be subject to a title transfer, similar to the private sale of a car or a boat. The state then could perform a background check at the time of transfer (and bonus: tax it!) without infringing on anyone's legitimate rights. Not sure why there would be any objection to that.