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If what you're saying is that you attempted to purchase something and the vendor simply took your money and refused to deliver it, then that is pure interstate criminal fraud (false pretenses).
If you used a credit card in connection with PayPal to make the purchase you can file a dispute with your credit card issuer within 60 days of the date that the purchase appears on your bill.
If you didn't use a credit card, then there's always this: http://www.ic3.gov/complaint/.