I have never sold one. I have traded two (one I made money on and one I lost) that I did not like, but right now I only have one that I would foresee swapping out (race gun in .45).
When I was young, and younger I heard many people express regrets over getting rid of a gun. Ever since I have tried very hard to do my homework, buy guns for a purpose, and USE 'EM. The down side is I keep running out of room to keep them.
Right now I can only think of two reasons I would sell out, economic hard times (need cash to live - once sold a motorcycle for that reason), or to cash out ahead of UK style bans and confiscation (however, I'm enough of a rebel I'd probably fight it too long).