I hate having to sell anything, but I can't hang on to all of it. So far, my biggest regret was selling my Marlin Model 60 - '80s vintage with the long magazine tube. It was the first rifle that I owned. I sold it to a member on INGO and he won't sell it back to me. I can't blame him.
I do not remember the brand but I sold a nice .22 magnum rifle that I wish I still had. It was many years ago and I was frustrated that my Brother could shoot and reload his .30 carbine cheaper than I could shoot my .22 magnum. It went to a good home though a farmer from Whiteland or Franklin was having varmints getting to his chickens and he needed something for that. Still I wish I had not sold it!
I've had several CZ-52s and SKSs over the years I picked up back in the "glory days" (sub-$100) and sold most of without even firing them... (Albeit at a profit) Now I don't have any of them, and prices have tripled...
Ruger 10/22 that I bought when I was a kid. I think it was around 1982. In the box all paperwork near mint condition. Just recently I purchased a 1985, very similar. I am happy, but still wish I had never sold the original.
I have sold guns because I didn't like how they worked mechanically, either by design or due to workmanship, and those I seldom regret. I do sometimes wish I had them back, though. "They weren't so bad," I tell myself. Or I tell myself, "I could have learned that trigger I didn't like if I had just worked at it more."
There was one gun I sold because, while I enjoyed it every time I took it to the range, I just didn't intend to carry it. That was a big mistake. I constantly regret selling my Bersa Thunder 380.
I will never sell a gun for that reason again. I have another gun that I will never carry (Taurus PT638 Pro SA) but I love how it shoots. I expect it will be a "safe queen" with occasional range outings for the rest of my life.