This could be a response to your ad or online auction posting, offering to pay with a 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 the cheque.
If you deposit the cheque, you lose. Typically, the cheques are counterfeit, but they're good enough to fool bank staff - when they bounce, you are liable for the entire amount.
Don't accept a cheque for more than you’re selling price, no matter how tempting the plea or convincing the story. Ask the buyer to write the cheque for the purchase price. If the buyer sends the incorrect amount, return the cheque. Don't send the goods.