Well now, interesting thread. Over the course of time going back to about age 13, I have bought/sold/traded enough guns to need a Cray supercomputer to keep track of it all!
Two most definitely fall in that "regret category"! (and of course a few also rans!)
Back in the 60s I had a very nice Model 92 Winchester carbine in 25.20. Saw something else I just had to have and a trade occured.
The one that I realy miss the most was a 1100 28 gauge skeet gun, ( I was shooting NSSA 4 gauges back then) That 28 with the weight cap was a very good gun for breaking clays. I never did have a 20 gauge I used the 28 instead and dide fine. On a good day I could even stay up or beat my 12 gauge 1100 SA skeet gun for score.
I then after a hard fight got my NYS pistol permit and gave up shooting registered skeet and concentrated on shooting and reloading handgun. A couple yeas later a friend asked if he could buy that gun and the MEC 600JR press as they were not being used. I had my eye on some handguns and foolishly let them go. I regretted it heavy duty a couple of months later I asked but he would not sell it back to me.
Just a couple months ago I started shooting skeet again after a 35+ year absence and not having that little 1100-28 just about gives me a attack of the Vapors.