As advised, you need to open an item not rceieved case.
A link has been provided for you.
You can do this one day after the latest estimated delivery date, so you may need to check the email you got from Ebay when you paid to check that you are not opening a case too early.
Really, the seller should cancel the sale which will trigger a refund and save you the bother of potentially waiting while you open a case etc. You could suggest they do this.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)