Mr. Hammond:
"This is the case" on the motor you sold that you were responsible for. The past owner has no responsibility in this equation. Unfortunately for you, it does not add up to to even a weak excuse. You made an error & you are responsible to correct it. Make no mistake, this is on your shoulders to work it out... not the buyer. In this case, I believe he WILL sue you & the consequences will get much worse. if you are banking on him dropping this, you are probably going to loose worse.
Then there is a moral obligation... the fact that you screwed him... inadvertently or not... you screwed him. "I am sorry" is not going to make him feel better. You will reap what you sow brutha. JMO.