Ebay can't help you unless you use the tools they provide, or you return with tracking if (wrongly) going down the DIY route, or your Proof of Posting receipt has a 16-digit 'delivery' code (which it will have if you returned the dress as a Royal Mail parcel and not Large Letter, and you bought the postage at the Post Office).
If you have a 16-digit delivery code, and one which proves delivery (check it out on RM's website) to the seller's Ebay-registered address, then open a Not As Described case and enter the number into it. Ebay can then force a full refund of your original payment. Return costs are yours to bear, unless the seller refunds them voluntarily or you take her to Small Claims court.
In future, don't go down the DIY route. Use the tools that Ebay supplies, so that you are automatically provided with a tracked returns label at the seller's expense, and one which will have the correct return address printed on it (the seller's Ebay-registered address, which is where you must return to in order to be refunded):
The Money Back Guarantee (on every listing, and in various other places across the site)
OR
The More Actions dropdown, beside each transaction on your Purchase History page
OR
The Resolution Centre (link at foot of any page)
OR
The Help pages (links at top and foot of any page).
@laura2502x
