I also sold a ball that refused to eat FT. She would not switch no matter what and the guy she was sold to was not able to feed live. I traded her out for an even better one that would eat for him.
It's better to fix the situation with another animal if possible, but if that's not possible, offer a refund with the return of the snake, or a partial refund if they want to keep the animal and can work out the problem on their own.