Batteries are expensive, and if you have a lot of gadgets, you probably are replacing them often. These semi-used AA Duracells cost a fraction of what a new 8-pack of coppertops would normally set you back.
Semi-used, you say? Pulled from children's toys that don't pass safety inspections, they're basically unused, and don't expire until 2016, but can't be sold as new. They're a mere $3.25 for 8 batteries on American Science & Surplus, compared to $14.99 for a new 8 pack on Amazon.