The scammer’s payment (typically a check or money order, but increasingly an electronic invoice sent via PayPal or another payment processor) is a forgery. In the case of a check, it is not real. It is only worthless paper. Your bank may allow you to deposit it, but the payment will not clear. Your bank will hold you responsible for the entire amount.
In the case of a PayPal payment, the scammer will either send a fake PayPal confirmation email or pay with a fraudulent payment source. Whether you return the “overpayment” via PayPal or a wire transfer service such as Western Union or MoneyGram, you will still be held responsible for all of the money involved.
Never trust a PayPal payment which is contingent upon you shipping the item before funds are “released” to your account, unless you are willingly using an escrow service.