Being a Ruger forum, I'll start with them.
P90 .45ACP.
It couldn't get through a magazine of ball ammo. I probably could've worked it out, but I had a chance to dump it back to the place I got it and took that opportunity.
Ruger Blackhawk .30 Carbine.
Nothing wrong with it, but I couldn't find a use for it or reason to keep it. So in that respect, it fits the definition of regret buying.
NAA Mini Revolver.
Tumbling bullets. The chambers had firing pin peening after a few shots. Not from dry-firing; from shooting live ammo.
A Sig P229 and P239, both in 357 Sig.
Guns were fine. Cartridge didn't do anything my .38 Supers couldn't do, but with less muzzle blast and easier reloading.
Just about anything I tried in .40 S&W.
I tried. I tried to give it a fair shake, but couldn't like it. I have kept one; a Browning HiPower. I kept it because it was the only one that shot good groups, and because it's a HiPower. They are hard for me to part with.
Bren Ten Marksman Special (.45ACP).
I regret buying it only because I traded it off after a year or so and they are now bringing a bundle. If I hadn't bought it, I wouldn't have traded it away and be kicking myself now.
Yes, I had magazines.
Walther P99.
I never got used to the operating system. It had sorta/kinda three trigger pulls, depending on what condition you put it in.
Remington 1100 20 ga.
Made in the 2000s/2010s, not during one of Remington's better quality eras. I never could get it working right.
Springfield Armory Inc M-1 Garand.
Not a government arsenal-made one. Newly-manufactured in Geneseo, IL. The quality was not at all to the level of the real thing, of which I have a few. I only got it because I was trading some things away and that was the only thing in the price range I had any interest in. I thought maybe I could use it to save wear and tear on my "real" ones, and let it be the one I let people shoot to try one out.