You buy a gift card off kijiji or a white van in a parking lot, the blame is on you for accepting an offer that is too good to be true.
Send out your "I'm sorry for your situation" email and social media post and publicly offer them 10% off their next purchase. Don't pay them a penny on that card.
And yea, send an awareness post along the lines of "don't purchase a second hand gift card unless it's 10 days old" or something. It's social media, make an annual "Scammer awareness day" and advertise it yearly as a way to bring in business.