A timely warning Simon. As it happens I am right in the middle of selling a car right now, through Gumtree, and although not the loan scam, I have had at least 3 people offer me the full asking price with a courier to collect, sight unseen, with some flimsy excuse for not being able to view it. Always done through PayPal interestingly.
I always tell them that I only deal with people in person and they have to pay cash, or else the courier has to pay cash at the time of collection. It always seems to go quiet after that. Funny.