This is super scummy mate. I would definitley side with you, The customer if they do this "Professionally or for income" Willingly bought a product that they believed they had the ability to change. To flash firmware on the card to allow it to be used by a Mac.
(Which we're not going to get on that subject because only certain cards are made to be flashable and used with mac.)
A they bought a working as described item, which was marked used, then used it. They fullfilled their side of the purchase agreement when they used the card (to convert it). If persay their case prevented them from inserting the card or perhaps they lacked the knowledge of power supply requirements that would be a different situation. But they failed at flashing their card. I would pursue legal action.
Supposing this is the case