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.