If it has been less than 45 days from end of sale, file an item not received dispute through paypal's resolution centre.
If it has been more than 45 days but less than 90 days and the payment was funded with a credit card, contact your card company and ask them how to do a chargeback since you didn't get the item you paid for.
If the item was located in the US, file a mail fraud claim through the U. S. Postal Inspector:
http://ehome.uspis.gov/fcsexternal/default.aspx
Non paying buyers deserve unpaid item strikes.
