How did you return it? Did you open an ebay return, and did you use a tracked method? If you didn't use a tracking number you cannot prove the seller received it and you can't insist on a refund.
What was the reason for return? If it was faulty/not as described you should have opened a return using one of those reasons and the seller would have paid for the return postage.
If you haven't already done so open a return now from your purchase history and add the tracking number to the case. Escalate if and when you need to.
