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.
Think you are mistaken....There is a ton of info and illicit use for empty giftcards. Don't sell them. You are most likely aiding crime.
Eventually you collect enough of a particular brand, and you can extrapolate BIN number, expiration date, and CVC patterns for each GC issuer. Collecting old giftcards helps you guess newer valid card numbers for a certain issuer. The hackers attack certain types like one brand of card who ship activated and another brand whoo doesn't require registration or address verification to use it online.. Many people think GC compromise only comes from physical or electronic breach. But the more sophistcated hackers simply spam thousands of online webstores with "guess" card numbers. I read it only takes a few seconds to generate a real, working card number with this method. Once they find one that works, they drain it. Having the most current BIN numbers means there are 6 fewer digits to guess. Knowing the expiration date patterns reduces the complexity of the problem even further. Which cuts down on the attempts required to generate a valid card number by several orders of magnitude.