Contact the buyer, let them know the mistake, and offer a refund if they don't want it.
If they want it, ship it. They can already return it for any reason so if they know the condition and still want it, you are in the same position as if the listing was correct. If they change their mind and file a return, they could have done that anyway and you're no worse off than had the listing been correct. If that happens, accept the return and refund as appropriate.
If they don't want it, refund and relist - this time correctly.
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