Did you send the money as a "gift" through paypal? Because unfortunately then you can't file a claim. I hate to say it, but then you would be out of luck. You should only use the gift mode for someone you trust very well (like family), and never for a business transaction. The gift mode waives the fees associated with the transaction on the end of the recipient. It is also clearly stated in the for sale/trade rules of the forum that the seller should not ask for a gift payment, or the thread/message should be reported to the mods. Have you tried PM'ing the seller?