Ok, just how long ago did this happen?

It is beginning to sound to me, like the customer opened a return request for Not as described.

You have not responded to it and subsequently, the customer has escalated it to Ebay.

If they have asked you from the case, to provide a returns label and accept the return, then you must do this!
If you don't, then the customer will be refunded from your funds and you will not get the item back.

This will be why the case is "Awaiting decision".  they are waiting for you to do something.

So, find the case, it should give you a link to the return, where it will give a reason for the return.
The customer MUST enter something here and a lot do this with pictures as well.

DON'T leave it!