I've lost roughly around $400 investing in RSGP sales. Some of it here, most on MMOExchange. It sucks, I know exactly how you feel.
The thing is, everyone says, "Don't go first! Use an OMM! etc." However, the thing is, during the trade, everything is fine and the deal goes smoothly. It's just, within the next 24 hours, they'll charge back on the payment through the "Unauthorized Access" option, (this option allows someone to charge back regardless of a "Gift" being sent). And since the product being sold is labeled as, "Digital Goods," there is no actual "tracking data" or "firm evidence" that the transaction actually took place. In which case, PayPal is obligated to return the money to the Buyer.
Source: (Many conversations with PayPal representatives, and personal experience)