Step 1: Take a deep breath!
Don't you just LOVE buyers like these!?! For as many years as we've been selling on eBay, you'd think we'd be used to this by now! In all honesty, there's no way of knowing how this will all play out in the end. Below are what I would do in your situation. No guarantees that it'll work, but it can't hurt, right?
There's obviously something fishy going on. First thing I'd do (if you haven't done so already) is take screenshots of the item that the buyer had for sale on their site. Just by her stealing your photos shows that she's up to something. Do you think she might have already owned a damaged dress, and was buying yours to "replace" it with on her own site? I suppose that it's possible that the buyer might had an identical dress for sale, but it just seems too coincidental. It's hard to know what some buyer's motives are in these situations.
What other feedback has the buyer left for others? Does she have a pattern of harassing sellers?
So let's say the buyer does file an "item not as described" claim. Per eBay's guidelines, before a refund can be issued for items "not as described", in most cases the buyer is required to return the item to the seller. If she does return the dress, I would think it would be easy to prove it wasn't the same one that you shipped since you have photographs of the original dress.
Again, it's hard to know what her motives are at this point. She may have resold the dress you had sold to her, and is trying (for whatever reason) to shake you down. If she did re-sell the dress on her own site, she won't have it available to return, correct?
I doubt that eBay will be able to help you in any way until the buyer actually files for a refund. They may be able to offer you some reassurances and let you know what the resolution process involves (if the buyer goes through with that) or what to do if they neg you. It sounds to me like the buyer already knows they don't stand a chance at getting a refund, and is hoping to play on your fears of a negative feedback. Who knows? She may be pulling this trickery on other sellers, and they've already complained to eBay about this buyer, and she'll get banned (it does happen!)
Believe me, I've worried myself silly, and lost sleep thinking about situations that pop up like this. When we work so hard to be GOOD sellers (top rated even!) and you get that one buyer that tries to ruin it for you....it's frustrating!
You should be able to get a phone number to call by clicking on the Customer Support link on eBay, then clicking the "Contact Us" button. They'll make you select a topic that you need help with, but you should see an option to call or chat with them (on the right-hand side.)
Try not to stress too much over it.
You've got great feedback, and if they do neg you, buyers won't be deterred from purchasing from you.Good luck and keep us updated!