The way the scam works, the buyer sends you more than your asking price. You are to wire the excess to a shipper selected to the buyer. You deposit the buyer's check, then wire the shipping fee to the shipper. Your wired money clears instantly. The buyer's check is fake and bounces. You owe your bank the entire amount of the buyer's check plus a bounced check fee and may face criminal charges for passing a bad check. The fake buyer and the fake shipper are the same person.
Variation: The buyer pays by PayPal. You get the notice from PayPal that the money is in your account. You wire the shipping fee to the shipper, then learn that the notice from PayPal was fake.