There's a few variations of the scam...
1. paypal payment goes through, goods get shipped and then payment reversed (claims item not as described / never received etc), seller loses goods & cash
2. Buyer generates fake paypal "payment received" emails and hopes the seller trusts the emails, seller loses goods and never had the cash to start with
3. buyer sends cheque for payment with extra "to pay the courier", seller passes goods to the courier (usually a friend of the buyer) along with the extra cash for their payment. cheque ends up being fake and does not clear, seller loses goods, cash for the item plus extra cash for the courier