Generally it's because the gun 'failed the audition', meaning I bought to try out and after shooting it I discovered I didn't like something. Accuracy, trigger pull, reliability---things you can't really evaluate while holding it in the shop. 20 years ago I was a frequent trader and had to let the old gun go so I could afford the new one, even if I liked the old one. Now I only sell guns that don't really suit me, or that fill a niche I don't really have a need for, or that have found themselves being dramatically outperformed buy a new acquisition. I went on a purge after my Dad passed away and I inherited his guns. I replaced several of my guns with similar ones he left me. His 22 magnum was practically identical to mine so I kept his and let mine go. He had a beautiful Hi Standard 22 target pistol so I sold one of my 22 autos to make room. As I get older and become more firm in my opinions about which guns I want/need/like I expect I will sell a few more.