Step one: contact seller (with pictures if possible) and ask for a refund.
Tell him your plan is to dispose of the item, and ask that he allow you to do this rather than returning it. Keep the item and the packaging.
If you don't get a prompt refund:
Go to the Resolution Centre and file an Item Not As Described dispute.
If the seller does not refund immediately, PayPal will require you to return the item to him with Confirmation of Delivery, which is expensive.
Once you can show Confirmed Delivery of the item, PP will refund you and go after the seller for their money.
If more than 45 days have passed since purchase:
Go to the credit card you actually paid with (PP is just the mediator) and phone the 1-800 customer service number. Ask for a chargeback. Have all your transaction numbers, dates and payment amounts handy for the clerk.