You don't seem to have any feedback for this problem. Your only neg is for an item which the buyer claims is not 'new', I understand that it is 'new-old stock'.
If your buyer had left feedback, and wanted to remove it , she would have to contact eBay to do so. You can't do this unless the feedback contains unpermitted vulgarity or something.
For the new old stock item, it is almost always best to tell an unhappy customer to return the item for a full refund. The customer can then decide whether the cost of the return is less than her unhappinees.
This is business. Not every transaction will be perfect (although we seller must strive for that goal.)
