Ebay seller protection policy says:
Signature receipt for valuable items – For items with a value of US$250.00 or more, proof of signature from the recipient will be needed to show that the item was delivered to the buyer.
http://pages.ebay.com/help/pay/paypa...rotection.html
So it's the seller's fault not requiring a signature upon delivery. You will win the case as item not received if you file a claim with ebay/paypal.