For a description of the way this scam is said to work see the report on the CBS news site and the secret service warning. In particular you should be VERY suspicious if your buyer is in Africa, and is giving you strange stories about having people in the US who owe him money pay for the car
update 1/30/2003. Another twist on this - there have been people from Dubai offering over the asking price but requiring your bank account number to transfer the money to. We understand the emails exchanged have several things in common - lack of proper punctuation, no capitalization, no periods, and use the abbreviation ASAP a lot.
Please be careful and use common sense. Consider using a reputable escrow company!