It's not always easy to find a box the size of a GBA box. If I'm selling a game and the buyer doesn't mind paying for priority shipping, I just send it in a priority box. However, if I'm trading and the person I'm trading with is sending media/first class, I just flatten the box and send it in a manila envelope. Most of the boxes I hold on to are bigger, so it'd just be ridiculous to send a GBA in a box that's far too large.