What @tressygirl said is good but you dont need Dublin to answer, in the live chat just write authentication speak to agent, it'll take 2 or 3 attempts but it'll finally come up. Whoever you speak to, Dublin, Manila, Albania or South Africa (loudest call centre I've ever spoken to) just ask to speak to the authentication department regarding (tell them your order number) they will then transfer you to that department and they are all located in Dublin, as long as you phone between normal UK office hours.
The people working in the sneakers department are top class (to the point I emailed the Head of eBay UK to praise them) and thats who eBay should copy for all their customer service agents. As long as you sent with the label you were provided they will contact whoever the courier is, get them to run a few checks (they contact the pick up store and the driver) and then if it is indeed lost you will be refunded a long with the buyer.
If you have sent it yourself using your own postage you will have to claim back from whichever courier you decided to use so hopefully you didnt do that.
