With overpayment scams, fraudsters play the role of buyer and target consumers selling a product or service. It usually works this way: The buyer "accidentally" sends you a check for more than the amount they owe you. They ask you to deposit it into your bank account and then wire them the difference. A deposited check can take several days or more to clear. When the original check turns out to be a fake and bounces, the victim is still on the hook to pay the bank back for any money withdrawn.
Fake checks can be used for any type of scam, so always wait for a deposit to clear before writing checks against the funds.
Here are some tips from Western Union to help you avoid overpayment scams.