I've sold and traded away many guns for lots of reasons. Sometimes I get rid of them because I don't shoot them well or they just don't have a niche in my safe. Other times, I've traded so I could upgrade. Virtually every time I've sold a gun specifically to raise money, it's been a sacrifice and I've regretted it. Here are some guns that I've sold and not regretted.
Ruger Security Six (cylinder would bind in DA mode)
Star Firestar (great little gun but nothing special)
Ruger Vaquero (thought I was getting into CAS but didn't)
S&W 66 snubbie (too many similar guns)
Charter bulldog .44 (light strike and often failed to fire)
Ruger P95 (bought it on a whim but it was too clunky)
I'm sure there are many others that I can't think of at the moment. Maybe later I'll list the ones I've regretted. I won't even go into the long guns...
To a much greater extent than most mechanical devices, firearms are terribly unforgiving of any overconfidence, complacency or negligence.