I sold all (well, both) my handguns chambered in .40S&W. Other than that, the only handgun I've sold is my very first one and I regret it. A friend kept asking me to buy it and I had the opportunity to upgrade to a nicer model of the same gun with night sights so I sold it and traded up. Wouldn't you know, I've never shot the "upgraded" gun as well...
Oh, and I sold a .30-06 Winchester Model 70 Classic Super Grade once. It was a gun that had been in the US Repeating Arms archives and I realized I would never shoot it so I sold it to someone who would hopefully appreciate it more.