i think everyone has given you the correct things to be aware of. if he wants to pay in cash i would go to your bank with him and pay it straight in !! just to be safe.
that car would be worth 12k+ euros to him if he is thinking of passing it on in germany.
not sure how he will get hold of £8k cash sterling in germany though, prior to his journey. if its euros definitely go to the bank before you part with the car.