The car was in an accident but I didn't disclose it when I sold it.
That's awful. Why would you withhold that information?
Some of the damage hadn't been repaired and I didn't want to explain that.
Having information about something you're trying to buy either lied about or withheld is seriously not okay. The person that bought the car is going to have to get things repaired and spend more money because of that. When you buy something, it's expected that you have all of the relevant information you need about the item or service.
It's no big deal because the buyer wasn't going to be happy anyway.
it doesn't matter if that will just make him more unhappy, disclosing the accident is needed. The person could have another accident because he didn't know important information when he bought the car.