Tell your seller to cancel the order by selecting 'cancel order' then the 'out of stock' reason, which will refund you.
If they don't know how to do this, send them here.
If no joy, you'll need to wait until the latest delivery date showing on the order has passed, and then file an eBay item not received claim. If a seller made me jump through those hoops to get a refund, they'd be getting appropriate feedback from me.