If I sell/trade one, it's because there's a reason for it - either I just don't use it anymore or there's a feature I don't like but didn't realize when I purchased it. Remorse, if present, usually fades fast.
To be honest, the only "remorse" I've felt in a long time is not in selling/trading but in "buying". I was starting to shoot a lot more rimfire. I wanted a Sig 1911-22 and found one at a great price. I even switched out the grips with some nice double diamond checkered ones - looks sharp. Unfortunately, it sits and I've only fired a few rounds out of it. The ammo is just still too hard to find. So, I do have remorse over purchasing a new handgun that I can't use. It's not the handgun - it's nice - just nothing to feed it. A sign of the times . . sigh . . .