I´m trying to sell my motorcycle online and received a similar offer. We´ve been exchanging emails for a few days now, even tough i knew right from the begining it´s a scam. I am now trying to outscam these guys somehow. will let you know.
Eyal Levy

"How the Counterfeit Cashier's Check scam works:


It starts when people from overseas posing as potential buyers of large items for sale on the internet, such as cars, ask the seller of the item if they can have an associate of theirs in the United States mail them a cashier's check for the amount of the item plus transportation overseas in order to expedite the process. They ask that once the check has cleared the bank, the seller withdraw the amount that is for transportation, and wire it to the buyer so he can schedule the shipment of the item.
When these cashier's checks are deposited into the accounts of the sellers, the banks let the customer know that funds are available in 24 hours, which is correct for a legitimate cashier's check. Feeling safely assured the bank customer goes through with the transaction. Then, days later, when the checks are found to be counterfeit the banks remove the entire amount of the check from the customer's account.
The scam works because everyone is assuming that it is a legitimate cashier's check, so it would be available in 24 hours. But it takes much longer for it to be realized that the check is not legitimate, and by then it is too late for the victims.
In some recent cases, before the details get worked out, the Nigerian car dealers will say that they have already sold your car and they need it immediately. Then there becomes a problem, the person that is buying from them is told their selling price of the car and to make the check out to the you, the US seller of the car, so you will be sent a cashier's check for that higher amount and then be asked to Western Union the difference to them for shipping and entry fees to Africa."

For more information check:

http://geocities.com/scamvictimsunited/

AND ALSO:

http://www.quatloos.com/nigerian419forum.htm