The scam works because the Federal Deposit Insurance Corp. (FDIC) requires banks to make money from cashier's, certified, or teller's checks available in one to five days. Consequently, funds from checks that might not be good are often released into payees' accounts long before the checks have been honored by their issuing banks. High quality forgeries can be bounced back and forth between banks for weeks before anyone catches on to their being worthless, by which time victims have long since wired the "overpayments" to the con artists who have just taken them for a ride.
Although this scam is in its infancy, real people have already been bilked out of thousands of dollars by it ? in some cases tens of thousands. The con has claimed victims in communities across the USA, so don't let your not having heard about it before lull you into a false sense of security. That the game is new doesn't mean it's not dangerous.
What You Can Do:
* No matter how sweet the deal, don't get involved in any sale where the buyer wants you to accept a check for an inflated amount and refund the overage.
* If you accept a cashier's check as payment for something you have sold, make sure it has cleared the issuing bank before you refund any money or surrender possession of the vended item. It may take two to three weeks for the banking system to determine the check is counterfeit, so even if the funds look like they're available (and even if your bank tells you they are), hold onto whatever it was you sold and the funds you received for it for three weeks.
* If you have been bilked, call the U.S. Secret Service at (202) 406-5572 or write to U.S. Secret Service, Financial Crimes Division, 950 H St. N.W., Washington, DC 20223. Also, call your state attorney general's consumer protection division.
Edited to say: The above was taken from the snopes.com link I posted earlier.