Surely I am entitled to a refund for defective items.
Yes, you were, but you may have blown it by opening the wrong type of return.
You should have opened a case under eBay's money back guarantee, which the seller would not have been able to close.
If you opened an ordinary return request, the seller can close it. You can't open two cases for the same item, so there may be nothing further you can do through eBay. Unless the seller states that they accept returns? If they do, but have refused your return request, you can contact eBay customer support to have your request upgraded to a money back guarantee case.
Otherwise I'm afraid you will have to look elsewhere for any refund, e.g. paypal or your credit or debit card issuer, depending upon how you paid. Good luck anyway!