Unless you do returns through an Ebay case, then you cannot escalate if the seller does not refund.
Doing returns your own way is a recipe for disaster, as seller knows you cannot enforce a refund.
You should always read the Money Back Guarantee and do a return properly.
Why did you return? Unless it was NOT AS DESCRIBED, a Private Seller is not obliged to accept a return, so even with tracked proof of delivery Ebay would not get involved.
