ALWAYS.....
Why not protect yourself and the other guy by writing a simple bill of sale. All it has to have is the make/model/serial number of the gun, a description of any flaws the gun might have and a statement that the deal is for X amount of money and the gun is being sold AS IS or whatever you want it to. Make two copies and you each can signand date both and each have a copy. If you're selling and the guy suddenly has buyer's remorse it keeps him from claiming the gun had some unknown defect and if you're buying it serves as your receipt that you acquired the gun legally (and unknowingly if it is a stolen gun). Who knows how the other guy got it? Unless it is a close friend, it'sALWAYS worth the extra trouble.JMHO.