I don't know if it is really linked to damage, but in "GT auto" you can rebuild the engine and increase (or rebuild) the solidity of the car.
I do this on used car, just after, you cant do it again, but after one race, the repair is available again.
For the engine, you can see if it is damaged/used (decrease of hp) but for the structure (chassis) i don't know
edit: for now i didn't see any decrease of the price because of damage (this can change depending of option, lvl??)