Any used item needs to be looked at carefully.
I am skeptical of any carbon frame where one can see the structural weave, (a number of frames have an outer layer that really is just for appearance- but how do you know?)
A good question.
One thing I can state is that the structural resin that holds the fibers in place is vulnerable if you can see it.
So, the more UV exposure the worse off. How much is too much? Who knows.
I think the later carbon frames where there is an opaque top coat all things otherwise being equal will be better off.