It is unfair, and bad selling, but no one can force a seller to send an item no matter for what reason they are claiming is reasonable.
For some consolation for you, as the seller quoted out of stock as the reason for cancelling, they will have received a damaging defect on their account. If this was a Private seller, it doesn't take too many of those defects for eBay to place sanctions even bans from selling, on the seller's account.
You can leave appropriate feedback .
@le_576997