Fraudulent check/cheques are used in a variety of scams such as advance fee letter fraud, overpayment and prize pitch to name a few.
Overpayment scams is the type of fraud where the person receiving the check/cheque is actually owed money for goods sold. The seller receives a counterfeit cashier's check/cheque, personal check/cheque or corporate check/cheque from the "purchaser" in an amount in excess of the amount owed; is asked to deposit the check/cheque and wire the excess funds immediately back to the sender/purchaser or the purchaser's agent or shipper; and, the deposited cashier's check/cheque is subsequently returned as counterfeit and charged back to the seller's account.
Anyone selling goods should be suspicious of any check/cheque, especially if it is for more than the agreed selling price. Consider an alternative method of payment, such as an escrow service or online payment service. Talk to your bank about the safest way to receive funds from overseas.
To protect yourself against this sort of scam, never agree to a deal in which the payer wishes to issue an amount for more than the agreed price and expects you to reimburse the balance. The scammers use a variety of excuses to explain the overpayment, but any such excuse should be treated with the utmost suspicion.