Someone responds to your ad or eBay posting with an offer to pay by cheque. At the last minute, the buyer comes up with a reason for writing the cheque for more than the purchase price, and asks you to transfer back the difference after you deposit it.
Usually, the cheque is counterfeit, but it'll be good enough to fool bank staff at first. You transfer back some money to the buyer before the cheque has cleared, but soon your bank contacts you to say the cheque has bounced. There's no way for you to recover the money you've los