IMHO that makes no sense. Yes, scammers can rewrite the magnetic strips. But since any card is rewritable, why would a scammer need thousands of them? Rewrite twenty cards, cash them out, rewrite them again if that's the ploy. No need for thousands of cards.
I looked into it once when I saw people paying upwards of 50 cents per empty card. I can't remember exactly what I searched, but it ended up leading to carding forums if you dug deep enough into the results. Do you really think people are paying $60 for 100 cards for arts & crafts? Especially when some sellers list what expiration month combos they have. That doesn't matter for arts and crafts or using as a putty knife.