If you want the dress back, you need to go back to the open case and provide a return label. You will have 3 days to do this or the buyer can ask Ebay to step in and refund them.
A lot of clothing sellers have a 'buyer pays' return policy so that, if the item doesn't fit, isn't quite the right colour etc., buyers can return without opening an 'item not as described' case.
"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)