Let's see if I have this right.
You bought a used part from an E-bay Seller.
You paid via Paypal but the part has not arrived.
USPS acknowledges that they were given the parcel.
Whether the Seller waited 11 days prior to handing the parcel over to USPS is irrelevant. There could be several reasons as to why he waited that long.
As I see it, the Seller is not responsible for this package once he places it with USPS so I fail to find any rationale for you to criticize the Seller over the non-delivery of the item. He did all that he could. The issue is now between you and Paypal. File a claim with Paypal. They will eventually refund your money due to the transaction not being completed. The Seller can file a claim with the Post Office.
Sorry about your luck with this transaction but things like this do happen and they have happened to me in the past. e-Bay has nothing to do with shipping of items and therefore are without blame here, as well. You could have had the item shipped via a courier service but that would have cost more. Under the circumstances, smearing the name of an e-Bay Seller on an open forum without any just cause is pretty pathetic.