I would never buy any used carbon parts that would be weight-bearing (frame, fork, seatpost, crank, etc.) So I guess that leaves carbon water bottle cages or bar end plugs. I admit that I do have a bit of a phobia about carbon breaking. The irony is that I have a carbon fork on my bike, because the manufacturer had a promotion where they gave the fork away for free with the purchase of a frame. That was 5 years ago, and everything has been fine so far.