Tell him to 'f' off (in the kindest manner of course).
If you wrote 'sold as seen' (and he test drove it) then that's what you have done. Just make sure he isn't the type to post a petrol soaked rag through your letterbox!
By the way, the MOT has nothing to do with a noisy gearbox. That would not be a failure as it's not covered in the test procedure.