Oooh this has always been a moral dilemma. If I am selling to an individual I tell them everything I know and everything I suspect, but then if I sell the car myself I'm not getting rid of it because of mounting mechanical issues.
I would be equally forthcoming when trading with a dealer, but my trades are usually too old and too many miles for a new car dealer to sell themselves, and they are just going to dispose of it somewhere else. Any in-depth information I provided about fried ABS computer, worn cv joints and the fuel pump I expect to fail within a year because of the noise its making, would not get passed along to the eventual buyer. Sad but true. I just pretend I am my wife and tell them the things that she would know about. ABS light on, gas gauge doesn't work below 1/2 tank, tires worn etc.
I traded a suburban that actually had a negative net value if I attempted to fix everything that was wrong with it. It showed up at a under $5k used car lot across town. ABS light still on, fuel gauge still broke, same worn tires, but priced 2.5 times what I got in trade. At least the buyer got a car that had been regularly maintained.