You can't start an item not received case until one day after the latest estimated delivery date.
Some sellers have a 5 day dispatch time, or even longer.
Check the email you received from Ebay when you bought your item.
It will give you a short range of estimated delivery dates. You should be able to open a case one day after the latest EDD.
"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)