The scam - you would receive a FAKE PayPal invoice sent by the thief. It would look real but if you were to check your PayPal account it would not exist. If you were to send payment to the shipper (actually the scammer), you would be out that money. (Were you to wire money? That would not be traceable.)
Here is how the scam works. The send a fake email that looks like it came from PayPal with your price plus the money for the "shipper" if they are also running that scam. If you actually check your PayPal account the money is not there and if you ask PayPal there is no pending deposit from the scammer. They will often claim that PayPal will not release the funds to your account until you have wired the funds to their "shipper" or provide proof of shipment. If you carefully examine the headers of the email message(s) supposedly from PayPal you will see they originate from other than PayPal's systems. If you do what they ask, you are out the money and/or the item(s).