If the guy who bought the OP's car was concerned, he should have opted for a detailed AA check.
Choice of attitude. I don't settle for relying on the "sold as seen, your problem now" answer to a sale where I've failed to accurately describe the item in the advert.
This boils down to opinion of morals. "sold as seen" isn't actually gospel when it comes to private sales. You are an individual and how you conduct yourself is down to you. You have a choice. If you justify the way you conduct yourself in private sales with "that's just the way private sales are done" then that is down to you, it's your
choice to conduct yourself this way.
I've made a choice about how I conduct myself in private sales. My opinion is that the onus is on the seller to accurately describe the item he is selling. Failure to identify a fault by the buyer at the time of sale is not sufficient for me to pretend that I didn't just sell a falsely advertised item. It does not validate my actions. "That's just the way private sales are done" does not validate my actions.
I may have innocently been inaccurate with the items description. Show me proof of my negligence (in this case, quotes from multiple garages which consistently describe the fault) and I will pay for my error.