Originally Posted by
cpach
It's harder to see structural damage to carbon frames so it's more risky to buy used equipment, and as with anything there are products that are poorly manufactured (in metal bikes too!), but if a carbon frame does not degrade under normal riding conditions and can have an indefinite lifespan. Carbon can be damaged by forces that would not damage a metal bike, as carbon bike frames are more likely to be damaged by impacts to sharp objects and crushing forces.
That said I wouldn't buy a frame with a known history for structural failure regardless of material.
I agree with what you have said.
The main concern for me is that building a quality carbon frame takes more skill and diligence than metal frames. It is easy to build a poor quality carbon frame with thin spots and inconsistent wall thickness and all of that is impossible to see once a frame is built without expensive imaging or other non-destructive testing.
Having said that, I think buying a 10 year old carbon frame may be as safe or safer than a new frame. If the frame has survived 10 years, your past the point where failures from mfg defects should have manifested. Consider it 10 years of non-destructive testing.