I trust you, but cut the cards.
I'm sure that most shop owners and employees are fine honest people, but if you went into someplace and Joe Smith the part time employee decided to buy the gun with no receipt to you, and you got paid in cash, and then he black markets the gun to some gang banger, and it turns up under a body..... that would be worst case scenario and if you were the first purchaser of the gun it's easy to find you, If you bought it used it may take a bit longer but eventually chances are someone is going to flash a badge and ask you some questions. You'd really want to be able to prove where it went at that point rather than try to convince a jury you had nothing to do with the crime or anything to do with the gun being there.