Guns I've sold: Texas Mac10, Norinco 1957 SKS, Ruger 10-22, Remington 700 22-250, Older S&W Police 38 special, Taurus Pt-99, Mac de Arms .38 snubnose knock-off of a S&W.
Of those guns, the only one I regret selling was the Mac de Arms .38 - It was my first gun, and a pretty decent knock-off of the S&W. I've never since been able to locate another one. I doubt that it's worth anything - I just wish I still had that gun.
Cars I've sold: The only car I've sold worth mentioning was a restored 1964 1/2 Mustang Convertible - V8 with factory 4-speed manual transmission. I drove the crap out of that car for 5 years, and it drove the crap out of my bank account just keeping it driveable. I have no regret at all selling that car!! In fact, it cured me of ever wanting to own a "classic" car, unless it's built on a modern frame with modern fuel injected engine, cooling system and modern 6 speed transmission. Some day that's exactly what I intend to do with a 1960-1962 style Corvette - give me that classic look on a modern frame with all new engine and components. They just don't make cars the way they used to.....and thank heaven for that!!
Motorcycles I've sold: 1976 Honda CB 550F - I restored it and when I sold it I was glad to see it go - don't want it back either. Kawasaki 900 Eliminator - sold that and don't want another. Basically, when I'm done with a vehicle, I'm done! They just don't last forever, and I don't like trying to make them last forever.