If seller cancelled for out of stock a refund would be made automatically, and payment would go back to source and may take some time.
However using that reason, seller gets a defect and still pays the selling fee.
Assuming you are correct and seller has not cancelled, you can get a refundusing ebay's 30 day money back guarantee, IT IS ON THE RIGHT SIDE OF EVERY LISTING.
As soon as past the estimated delivery date, you can open a NOT RECEIVED case and seller has 3 days to refund, if not you escalate the case to ebay.