I've only sold/traded 3 guns.
One was a .357 revolver I sold here, an early-model Ruger Security Six. It was a sweet gun, but the early models had ridiculously flared grips. They only made about 100,000 before changing the design of the grip, so after-market replacements are virtually impossible to find. It was too big for my hands, and the timing was also off just enough that it shaved lead 1 round out of 6. So, no, don't regret selling that one.
One was a double-barrel S&W 12ga. I spent a year trying to shoot trap with that gun, and just could NOT hit the birds. It didn't fit me, I didn't fit it, something was amiss. After a solid year of practice, I gave up and sold it to a family friend. I don't regret selling that one either.
The last was an Izhmash single-shot break-action in 7.62x39. Imported by Remington, I think. It was a cute little gun, but if you used lacquered steel ammo, the casing got stuck every. single. shot. And it really wasn't terribly accurate. My father-in-law REALLY wanted it, so I traded it to him in partial payment for a Santa Fe m1903. And I don't regret the trade.