What criteria do you use to determine the fair, good, under and over value of a car?

I have owned 26 corvettes in my life. So the main thing about good, is a good running engine with average miles, good to nice interior. A straight body with a few small flaws, that I myself could fix. I have bought good 1973's that had some wear, but was OK, to be seen in. I once bought a 1967 convertible with no interior, and no paint. It was found in a body shop that a guy dropped off many years ago that added ZL-1 flares, hand made front flares, 6 tail lights , missing the lights, and shaved front parking lights. A friend bought it to get the speed control from it and sold it to me for $3100, this was back in the '80's. I painted it, put new interior in it, new tail lights. It was a gorgeous customized 67 , red with a big block hood with a white stripe. Everybody in Virginia, and North Carolina loved it. I auto crossed and showed it for 4 years, and later sold it for $14,000. Been sorry ever since. Most of my corvettes were new so I didn't have to do anything to them except add loud exhaust , and lots of chrome under the hood. I owned the very first C-5 , 1997, in Richmond, Va. One of the first 500 unveiled to the public. Sorry , I sold both of these cars.