Some folks seem to have the idea that if they just sell the gun "off the record" with no paperwork, it can never be traced back to them, so no sweat if the buyer is a stickup artist or a serial killer. Believe me, if the case is big enough and there is enough pressure, you can bet the police, FBI, Secret Service, ATF, etc., will work to trace that gun to everyone who ever handled it, and then ask questions of each owner. And if you believe you can just blow them all off by lying, or saying you forget, think again. You don't want to be on the receiving end of an "intense investigation".
(Hi, Evan Price. Your advice sounds good. Have you ever tried it on the FBI or ATF? My advice is don't.)
Jim