Have you tried doing it through the resolution center?
http://resolutioncenter.ebay.ca/
If that works and the seller doesn't reply to the return request, you can ask ebay to step in after a certain amount of time. (3 business days?) Or if the seller tells you to return them, ask them for a return label as the Money Back Guarantee does state a seller should pay for a return if the item is not as described.
But - It won't be possible for the seller to send a return label and shipping to China with delivery confirmation would cost $50+ so if the don't refund you without requiring a return and/or do not pay for the return (before you send the item back) then you can ask ebay to step in and tell them that the seller refuses to pay for the return shipping. You will likely be refunded without being required to return the mittens.