You say you have already returned the dress, that is your first mistake.
Returns have to be done through an ebay case, and you use the dropdown menu to start a return.
Is the seller a private or business seller? If a private seller they are not obliged to accept change of mind returns (does not look right), only business sellers have to accept change of mind returns.
I hope you returned using tracked mail, as you need to open a case and add the tracking to the case, as that is the only way you can get a refund from buyer if they do not refund voluntarily.
For information paypal only deal with not as described cases, not change of mind. You should always open an ebay case, as returns are not dealt with by messages.
On every item on right side Is the ebay 30 day money back guarantee, I suggest you read it so thst you know what to do if you receive no response from seller.