Sometimes we just lose money. I have dealt with cases by "calling the bluff" of the complainer and offering a full refund on return of goods. On two occasions the genuinely unhappy buyers did return the items but usually they decide to keep them. I don't entertain partial refunds unless I am certain of the facts.
Be very careful because eBay's system can surprise you be issuing a full refund instantly if you select the wrong button.
Antique mirrors often have replaced glass as they cease to work as mirrors when too tarnished. I think she was hoping to buy an antique mirror with its original darkened glass, a seriously expensive item now.

If you offer a full refund upon return, she might just decide to keep it as she probably just wants a discount.