no real huge regrets yet. i've only sold two cars so far::
--1990 7-up mustang lx 5.0 vert. was in shitty condition, bought it for $3,500. sold it 5 years later after only putting less than 5k miles on it for $2200, after sinking about a grand into it. it had rust, was an auto, with 2.73 gears
--'99 sentra sold last year for $2,500 with 115k miles on it. mechanically perfect car with everything working on it--wife made me sell it, no huge regret.
--'90 grand prix. my grandfather helped me buy it after college (in 1999) as a daily driver for $3,000 with 75k miles on it. after maintaining it beautifully, and fixing a head gasket at 150k miles, my wife demanded i get rid of it (let me get the gto just to get rid of the grand prix). i gave it to my grandfather, and he absolutely loves it. he's driven it for the last 3 years and it has worked flawlessly for him
our '05 explorer kind of sucks, i'd love to trade it in on something better, but its paid off.