Regret selling any pistol? Not really, well maybe for a day or two. That would be a stainless Colt Series 80 MKIV Government. Dressed up with the prerequisite match goodies i.e. adj. combat sights, Smith Alexander flat magwell, Safariland grips, McCormick trigger, Svenson Ambi Safety, Wilson beavertail, front strap checkering, etc except it had the original barrel to keep the slide frame fit loose (for reliability in my mind at least) , as it was a carry piece (yeah, that was a previous life).
What I really, really regret was requesting a gunsmith friend (?) sell a completely built, stock Colt Combat Elite, and then he disappearred from the face of the earth (the same previous life 17 yrs ago). So technically I didn't "sell" that pistol and sometimes still think of that piece to this day.