I've sold or given away 3 firearms in my life. My reasons are:
Highpoint c9: "I don't use it anymore, and you need a defense handgun, even if it is fat, ugly, and mongoloid."
Highpoint 45: "Was my first .45, and now that I have my xd45c, which is better in every way, I don't need it anymore."
R870: "I think it is fitting that I turn this into a shotgun wedding! Congratulations!" (In truth I wanted a new r870 with a prettier walnut stock.)