It was a private sale, he checked over the vehicle before cash was exchanged, and a deal was made. It's a 40 year old truck, what does he expect? You can't be held responsible for issues he failed to check for.

Short story, years ago I sold an old Xbox on CL for $50. It had issues, and I was clear to point them out in the ad. I invited the buyer over to inspect it and play some games before he committed to a buy. He declined, asking for me to meet him in a parking lot someplace. I agreed. We met, cash was exchanged, and the following day he calls me complaining I sold him a faulty xbox. He complained about the issues I made VERY clear in the ad. He proceeds to blast me with text messages and phone calls. Easy solution for me: Number blocked. Done deal. Sorry.

Once that xbox was in his hands, I don't know what he did with it. Maybe he was in an accident, maybe he dropped it when he was getting out of his car, maybe he had loftier expectations of a video game console built in 2001, maybe he just didn't read my ad carefully enough. Who the hell knows, but none of it was my responsibility, and he had every chance to test it thoroughly before committing to the buy.

In your case, a lot can happen in 6 days. Maybe he drove it through a carwash and damaged a seal? Maybe a light burnt out while he was driving it? Maybe he has a heavy foot, or was driving down a long hill, or carried a heavy load and is experiencing brake fade? Who knows, but none of that is your responsibility. Especially on a truck built in the early 80s.