Can I trust a carbon-fiber repair?
It might be hard to tell by closely inspecting the repair how well the work was done, or for that matter how badly the frame was damaged. I would ask to see pics of how the damage looked prior to repair. I would inquire about his experience repairing carbon frames, see if he has any testimonies from other customers. I assume he buys cracked frames and repairs them to resell as a business or side job? It might be great work, but a testimony would help me decide. I would also dig around a little to figure what the value of that build would be if the frame had not been damaged.Maybe you can find a similar build priced well that hasn't been damaged? 
Take it for a good test ride. See if he can offer a return if anything goes wrong at the damaged stay within a set time frame. Try to talk him down? These are all things I would try. It really just depends on how much you want the bike and how much you trust the guy you are doing business with.