You had 30 days to open a return case using the Ebay Money Back Guarantee.
The seller has used delaying tactics and you are now out of time to do anything through Ebay.
If you still have the 'wrong' dress, you could try to recoup your loss by selling it yourself.
"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)