A thread revolving around this topic came up in the past. The Nigerian's are notorious for various types of fraud. This describes one scam that if my memory serves correctly, sounds like one of our DRNers encountered while selling a dirt bike.
Counterfeit Cashier's Check Scam
Although the Nigerian fee scam is becoming more well-known, a new type of overseas financial scam is making the rounds. While the Nigerian scam targets victims' greed (who doesn't want a large sum of money?), the counterfeit cashier's check scam targets the average person who's selling or auctioning goods on the Internet.
Here's how it works: You put up a large item for sale (a car, for example) or auction. Then you're contacted by someone from overseas pretending to be interested in buying the item. They ask you if a friend in the United States can mail you a cashier's check to cover the cost plus transportation fees.
You may see no reason to object, since, unlike personal checks, cashier's checks cannot bounce because the amount is paid to the bank when the check is issued (similar to a money order).
After the cashier's check is deposited, you are asked to wire the transportation cost to the buyer so he can arrange for the transportation. It's only when the cashier's check is discovered to be counterfeit that you realize you've lost a large sum of money.
There are also variants where you ship the item and lose everything.
How to protect yourself: If you receive a cashier's check, call the bank that issued it (the bank's name will be printed on the check) and ask them if they authorized that check.