To me, it sounds as if the ebay reps are fixated on the fact it is a second hand item and was described as second hand in the ad.

I am not sure what the seller has said, maybe that you expected new and she had described it clearly as second hand.

I do think you need to ring again and ask to speak to a supervisor and I think you have to focus on the fact the frock has significant damage that was not described in the ad and not shown in the photos.

Don't even get into how it is a stock photo & that may be against ebay policy, don't side track things. If they have a look themselves at the ad while you're on the phone, then you can say the photo is not a photo of the one you got because yours has fading, fallen hemline etc

Can you take any photos of the damage and offer to send them through to show what you mean?

Good luck.