On Monday, 19 August 2019 18:58:06 UTC+1, Jim GM4DHJ ... wrote:
guy just came to my door tonight with £12000 cash to buy a car
he had viewed the other day...I told him that he would have to
come with me to the bank tomorrow to state where the money came
from...was I right to do that or should I have just taken the
cash ? .....what would have happened...

Your bank would have asked you where the money came from. Presumably you would have a copy of the advert and the car sale/transfer forms to substantiate that, if asked to provide it.

Start doing it too often and you run the risk of your account being frozen for investigation.

Owain