Send the buyer a polite message to check through their purchase history again as it would seem,  they have contacted the wrong seller.

Do you have any tracking to show your item you sold to her is showing delivery,  or still in transit?   Do you write your name and address on the back of your items?  Always wise to.  If so,  ask her to check this. That may help you prove it's not your item.

@texol-stars