DonT· Deplorable bitter clinger.
You don't need a sales receipt. But in case you sell a gun or gift a gun, you would just write a memo of record to yourself, that on such and such a date, you gave/sold this gun, with this serial no., to such and such a person. File it away for just in case.You sell gun person uses it in a crime if you bought gun in store serial number comes back on you.
With no proof you sold it your a person of interest.