People can check if there activated, so buy an activated one and switch it out before you complete the sale.
You could sell them working, activated but write down the number on the back. That way, they can check them to make sure they are activated with the right balance, but you can use the number on the back to order shit online before they would get a chance to use them, preferably right after they hand you the money. Good way to double dip, but people only buy giftcards on craigslist for about 50% of their value, maybe 75% if you're lucky.