I've sold a lot over the years, guns I didn't care for, guns I ended up with duplicates of, ect. I don't regret selling those.
However, during a bad streak of luck that wiped out my savings, I was forced to sell some of my favorite guns to pay bills. And that hurt. Badly. I totally regret having to sell these.
Colt SAA in .44 special with a 7 1/2 inch barrel.
Browning Citoris in 16 gauge and 20 gauge.
T-series Browning Hi Power
DWM Artillery Luger
Sadly, I've never replaced them. I made a significant amount of money over what I had paid for the guns, but now in todays market I can't touch them for what I sold them for. The only one I've actively searched for I can't find in comparable condition - the Colt in .44 special.
I guess when I feel I can afford them and already have everything else I want, I'll start working on replacing them.