Ask the seller to cancel the order now citing out of stock as they clearly do not have the item to send to you, this will fully refund you.
If they don't then you need to wait one day after the item's last estimated date for delivery and open a case for item not received.
Go right of the item purchased, and from More Actions select.... I Have Not Received My Item.
If you are not fully refunded within 3 days, you must escalate the case on day 4 or within 30 days from opening the case, and eBay will step in and force the refund.
Do not close the case early for any excuse or promise the seller may give, and do not forget to escalate.
@immense-66