I regret most of the things I've sold, with a couple exceptions. I had a Kahr PM40, and it was the second most unpleasant gun I've ever shot. With the flush-fit magazine, it tried launching itself from my hands no matter how I held it. It was much-tamed with the extended magazine, but that completely destroyed the added conceal-utility the PM series has. Essentially, the pistol was fair-useless unless I made it less concealable. I sold that on a loss, and never looked back.
The other exception is any gun with the Taurus logo on it, with special attention given to the worst gun I've ever owned: A Taurus Tracker in .44 Magnum. It wasn't that the gun was poorly made, because it wasn't - that honor goes to the Taurus J-frame clone I had around the same time. It wasn't that it was made with cheap materials, because it wasn't - I had a Jennings J-22 at the time, shot thousands of rounds through it, and never broke it. It also wasn't that it had a bad trigger, because it didn't - again, that honor goes to the Taurus .38.
No, that Tracker was the worst gun I ever owned because it was unshootable. Despite the ported barrel, despite the "recoil reducing" grip, despite padded shooting gloves, there was simply no way to shoot that gun and not have it attempt to amputate your thumb while the front sight aimed itself straight for your forehead. I have an Airweight J-frame. I have a Redhawk .44. I have a single-shot -.454-. Nothing I have found is anywhere nearly as brutal as that Tracker was.