I am selling my car and posted on Autotrader last weekend. So far I received no less than 4 scam emails from crooks trying to part me from my cash. Fortunately, I did my research and came across this excellent explanation from Jim at Autoshippers UK. I am trying to summarise it here but please read his comprehensive post.
The typical scam involves a "buyer" purporting to be a garage wanting to pay the price and pay you extra for you to pay his shipping agent directly. Say you sell your car for £5K and shipping costs £500. You accept his offer, receive a cheque for the car plus extra for the shipping (£5.5K). Your bank informs you that the cheque has been paid to your account. You pay the shipping agent as instructed for £500, usually through Moneygram. A few days later, your bank informs you that the cheque has bounced and withdraws the amount from your account. Leaving you with having paid "shipping fees" of £500 to some unknown untraceable "shipping agent".
You have been warned.