I've owned 20+ vehicles and the only one I regret selling for what I sold it for was my 2002 Civic EX coupe 5 speed. The key words here are "for what I sold it for". It wasn't anything special, but it was a nice, comfortable, well maintained car that got 40 mpg and I traded it too cheap.
Everything else I've gotten rid of I feel like I got what it was worth, or got my money's worth out of it. That civic got traded towards a 2008 Si in a moment where my heart won out over the brain... The dealer listed it for 2.5 times what they gave me for it and was sold the next day...