Seller sells an item, then finds they don't have the item to send you (e.g. damaged, sold elsewhere, lost) the it is the Seller who cancels using the reason: The item is out of stock.
Their account will get dinged for this, because selling an item you don't have is bad practice. Don't let them talk you into cancelling when you actually want the item! They try to do this to avoid the ding.
For your refund, be patient until delivery date has passed then open an Item Not Received case. If the Seller still doesn't refund you, wait for the day that eBay can step in to help (in case details) and call Customer Service during a work day, best after 8am if you can.