Can I trust a carbon-fiber repair? https://www.kijiji.ca/v-road-bike/city-of-toronto/felt-z4-carbon-road-bike/1497484952

That's the bike, I'm just concerned about the carbon frame repair.

Seller said he did it himself and has done many frames before, and can guarantee its lifetime in writing.

Are there any specifics I should look for when I examine the bike ?

How do I know it is safe to ride ?
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.