Your in a tuff situation. Only you can decide. How would you feel if it happened to you? I know I would be pissed!! I would ask the guy how it happened. I would then ask the shop where the boat is located to check the motor with a scanner (if applicable) to see if in fact there may have been driver error i.e. over rev, extended wide open, water temp, oil temp etc. etc. . See if the scanner story and the drivers story match up and then make your decision. Usually a good mechanic can look at things and have some kind of clue what went down. Good luck!
P.s. You may want to contact a qualified OSO member in the area to do a little mechanical look. If the buyer didn't do anything wrong, there shouldn't be a problem.