Purchasing a part from someone online is and easy practice.
Here are the simple steps:
1: Seller advertises said part.
2: Buyer contacts seller about said advertised part.
3: Buyer and seller hammer out a deal.
4: Buyer sends payment.
5: Seller adheres to deal and ships/delivers etc said part.
6: Buyer obtains part and everyone is happy.
It looks to me like steps 1, 2, 3, and 4 were completed. I think the deal got stuck on #5. That would put the seller at fault, and the seller should have instantly refunded the money if he couldn't live up to his part of the deal.
Oh, and by the way, I sell daily on Ebay. The paypal commision gets refunded when a payment is refunded.