First contact the seller and explain your problem.
If you have not paid and he has not shipped, the transaction can be voided, either by a Mutual Cancellation or by an Unpaid Item Dispute.
If that doesn't work , you have paid (check your Paypal!) but it has not been delivered, you can file an Item Not Recieved Dispute. However, if the seller used tracking and the tracking show delivery you will not win.
If you have paid, and the item has been delivered, you can file an Item Not As Described Dispute. You will have to return the item with Signature Confirmation of Delivery to the seller. This will be expensive, but not $700 expensive. When you can prove delivery Paypal will refund you.