Usually, if an item fails authentication, a refund is made automatically,
Have you had an email from Ebay stating the item has failed authentication and you will be refunded?
If that is the case, you may just have to wait for payment.
Refunds can take several working days, longer if you paid with a credit card.
So, when did you hear the item had failed authentication and have you been told a refund is being processed?
I probably should ask what sort of case you opened? If you opened a return before you even got the item, that may confuse things!
"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)