Ireally hope that when you returned the item that you used a service that includes Proof of Delivery.
If not you cannot prove to Paypal that the item was in fact returned. And your refund in in the hands of the seller alone.
OTOH, if you backed your PP payment with a credit card , you have the option of going to the cc Customer Service for a chargeback. Mind you, they may also want to see Confirmation of Delivery to the seller.
Fingers crossed.