There are multiple gift card reseller sites that buy your gift cards and resell them at discounted price. I was wondering how these sites prevent sellers from reusing the pin and card number after it has been sold (you can use the pin and card numbers online, or even at the store without the physical card).
All the giftcard buying companies (like cardpool) are located in California because In the state of California all retailers that offer giftcards must refund giftcards for cash value or issue a new giftcard at no cost (California Civil Code Section 1749.5(b))
there is a 24-48 hour wait period for gift card buying companies to pay you.
So what they do is either get a cash refund or new card from the card that you sell.
The balance is determined by the card number and pin. On a physical card, this is stored as magnetic stripe data, in which case it can be captured and written to another card by a malicious person, however the balance would obviously stay the same, and couldn't be reused over and over. Online, The transaction is processed by a third party, or the business the card is for. Regardless, Somewhere in some database it says card xyz has a balance of 100.0 and when it is used, there is likely some server side side script that changes the value in a database making the card's value decrease, so it's easy to say after card xyz has no balance, it will no longer work.