Most consumers use these sites to get cash—though usually at less than the gift card's face value—or to swap for other gift cards that can be used to buy food and other necessities. But others are approaching shoppers directly, gift cards in hand. I experienced this firsthand while scouring the stores for bargains a few days after Christmas. A woman approached me in a department store, asking if I could use a $25 store gift card to buy the sweater I was holding in return for cash. She said she had recently lost her job and needed the money. When...