I will keep the first/last name of the person I sold the gun to and the date I sold it in my own files though. That way should the ATF ever come knocking, I will just say: "I sold it to First Last on 2/22/13, who had a valid MN drivers license and a permit to purchase/carry at the time". They can figure out the rest.
Of course I rarely sell guns. The last one I sold was to a well known state trooper

