The following are in no particular order and represent guns that I have regretted buying because of the guns themselves (functionality, value, etc). There are several safe queens that i might regret buying now because looking back because i haven't used them a lot, or at all, but I haven't included them here.
Bersa Thunder - two have broken. Pull trigger nothing happens. One I returned for lifetime warranty. The other I am keeping to get fixed under warrant because I can't return it to wholesaler like the first.
Ruger LC9 - horrible trigger, bad customer welched on gunbroker, sold eventually for less.
Ruger LCR - not bad trigger, way too light, very unfortable to shoot. sold it.
Remignton 770 (gun was fine for the money, but couldn't get it to "fit" right for eye relief)
KelTec P32 - carried nearly 24/7 for many years, but shot comparably little. Light, but long (and thus dreadful) trigger. Unfortable. The more I shot it, the more proficient I became, but the less reliable it became. It was always bad with hollow points and then started failing to jam with ball ammo I had fired hundreds of rounds successful. Now I would do Ruger LCP II or LCP Custom.
Walther P22. Bought it because I wanted a more "tactical" 22 that was more like a regular gun rather than my Ruger Mark III Competition (sights kept coming loose, but I don't regret that one yet). Wanted the P22 so I could have threaded barrel before SAFE kicked in. Gun is "meh." I would try the Ruger SR22 if I had to do it all over again.
Colt 1911 Anniversary Edition of 2011. Stripped down, GI model, still unfired in the box. Paid full MSRP as it was hard to find, Looking back, I would have settled for any Colt Government Model, like the one I bought in recently from an estate. My only regret is that I (had) to overpay at the time. Same is true of my first Colt AR, but I don't regret that.