Royal Mail Special Delivery.
Keep Reciepts.
Wait for the dispute resolution to kick in in a few days time – they will instruct you to return it (probably) after they intervene and they will instruct the seller to refund on reciept (probably).
That is if you have articulated your case (and the facts) appropriately enough for them to take your side.
When returning make sure everything is sent INSURED (at value) and is TRACKED. That way, if they do not pay up (refund) you can prove they got the goods, and ebay will pay you and pursue the seller. HTH