Can I suggest that you read eBay's money back guarantee policy, see: https://pages.ebay.co.uk/ebay-money-back-guarantee/
This will explain exactly what the buyer and seller are responsible for. Basically, most instances of loss or damage in transit are the seller's responsibility to refund the buyer. The seller may be able to claim compensation from Royal Mail or the courier, depending upon the delivery service they used, but they can't delay refunding the buyer until they do this.
Try to read eBay's guidance in full. There are all sorts of points that many buyers don't even realise - for example that if delivery is attempted but the buyer is out, eBay treats this as a successful delivery and the item is no longer covered for non-delivery under eBay's money back guarantee. Think about that before you buy anything of value here!