Okay, you got rooked.
EBay's Money Back Guarantee covered you for 30 days after delivery. The seller would have been told to send you return postage and when tracking showed it returned, you would have been refunded.
BUT.
You sent it on your own dime. You won't get that back.
And the eBay Money Back Guarantee is past the deadline for Disputing.
All is not lost!
Go to your Paypal account page.
At the top of the page under Tools is the PP Resolution Centre.
You can open a Dispute there for 180 days from Payment.
The reason is Not As Described.
The first suggestion is that you Contact the Seller.
You did that, move on.
You will be told to return the disputed item to the seller* with tracking.
You did that.
Or did you? If you did not track the shipment, you won't get a refund.
If you did, enter the tracking number in the Dispute.
If the tracking shows the seller has it back, you will be refunded.
*This is why you go to eBay first. Usually eBay makes the seller pay for return shipping.