I looked over my gun sales history logbook...I tend not to keep guns around when The Next Cool Thing arrives...so most all of them were sold without looking back.
Some however stand out: Walther PPS M2. I thought this was the bee's knees, and was the pistol I took to my first "real" training, a private coaching session with this Gabe White dude out in Portland, an up and coming trainer in Clackamas OR that shot USPSA AIWB. You might have heard of him. Anyway, I realized it was heavy as a boat anchor for a slim 9mm, and after the non-reversible mag release inadvertently released carrying it, twice, I realized a single-shot automatic was less than optimal for a carry gun. Sold it for a HK P30SK V1.
Which didn't work out either. I could never get my head wrapped around LEM. It was sold along with my VP9 when the Gen 5 Glock 19+SCD arrived in 2017. I don't miss it, at all.
Some honorable mentions, two Glocks, oddly enough: Glock 43X. I could never get used to how snappy this felt to me. It was just a pain in the butt to shoot. It went for my first P365XL, which in retrospect, I should have kept. Glock 26. I shot this gun 200 rounds, and owned it 15 months. It carried about the same as my G19, and realized it wasn't doing much for me. So it was traded for my wife's original P365. Which is another one I should have kept, I guess.