The scammer will post a vehicle online for well below market value. When the buyer contacts the seller, the seller tells the buyer he will arrange for shipment of the vehicle once the funds are transferred. Once those funds are transferred the "seller" breaks all contact and the buyer is out of the money.
In this case the seller is legitimate and the buyer is the scammer. The seller posts a real car for sale and is then contacted by a prospective buyer. The "buyer" then offers to send a cashier's check for the full amount plus additional monies for shipping costs. The seller is instructed to deposit the check into their bank account and wire the overage to the "shipper." When the transfer is picked up the "buyer" breaks contact and the seller is left responsible for the missing funds and the fraudulent check.
Scammers will often suggest the use of fake escrow services that will hold the funds involved until the transaction is complete and both parties are satisfied. The seller will offer to ship the car and tell you there is no risk of fraud due to the escrow service, but once the funds are transferred the seller again breaks contact and the buyer is out of the funds.