I sold several when I plain and simple needed the money. Those were hard decisions, but I accept them. Among the guns I sold to obtain other guns, however, there is one significant regret.
My Colt Series 80 blued Government Model (serial #FG66500): If any of you runs across it and wants to sell it, I'm the one to talk to. If you don't want to part with it, you've got a fine pistol on your hands. I just hope whoever has it is treating it nice. The CZ75 that took its place could not fill that .45's shoes.
I've traded off prettier, rarer, and more expensive guns, but none like that one. I just get attached to a pistol once I've carried it for years. When the bluing wears thin under the gripping position of your own fingers, you stand a good chance of valuing that gun above its market worth.