The Bait: A response to your ad or online auction posting, offering to pay with a cashier’s, personal, or corporate check. At the last minute, the so-called buyer (or the buyer’s “agent”) comes up with a reason for writing the check for more than the purchase price, and asks you to wire back the difference after you deposit the check.
The Catch: If you deposit the check, you lose. Typically, the checks are counterfeit, but they’re good enough to fool unsuspecting bank tellers; when they bounce, you are liable for the entire amount.
Your Safety Net: Don’t accept a check for more than your selling price, no matter how tempting the plea or convincing the story. Ask the buyer to write the check for the purchase price. If the buyer sends the incorrect amount, return the check. Don’t send the merchandise. As a seller who accepts payment by check, you may ask for a check drawn on a local bank, or a bank with a local branch. That way, you can visit personally to make sure the check is valid. If that’s not possible, call the bank the check was drawn on using the phone number from directory assistance or an Internet site that you know and trust, not from the person who gave you the check. Ask if the check is valid.
Courtesy of http://www.onguardonline.gov