I have sold or traded countless firearms, and then had some degree of seller's remorse. This being the autopistol section of THR helps me narrow the list, as most of my regrets have been with revolvers.
I wish I had kept my first Seecamp LWS-32. I sold it before the prices went crazy, and did not buy another until the prices moderated again, years later.
I miss my SIG P220, with the heel-clip "European-style" magazine release. This was my 24/7/365 carry/duty pistol from 1991 to 1993. I sold it to finance a P220 with the so-called American-style mag release, only to find that I really did not like the newer type, as the whole grip frame seemed to have been changed a bit, causing the new P220 pistols to point low, and just not feel right. I reverted to revolvers and 1911s, not returning to SIGs until 2004, with the P229.
Two 1911 pistols: a Colt Ace and a Series 80 "Classic Government" Model; these were casualties of my two large purges of excess 1911 pistols, in 1991 and 2002.
I miss my pair of HK P7 pistols, from the original P7/PSP era.
Of these, I reckon I miss the Colt Ace the most. I got it for a very good price, due to some minor corrosion on one side of the slide that had been polished away, which affected its collector value, but not its value as a fun, reliable shooter. At least I did not lose anything when I traded it for the above-mentioned P220 in 1991.
My silliest sale/trade was the Seecamp. I did not really need the money for anything, and I almost instantly realized it was a mistake. I then had to wait until the period of low availaibilty and high asking prices was over, to get myself another one.