For decades, Girl Scouts has used cookie sales to raise funds and teach scouts about entrepreneurship. This year, thanks to the Raspberry Rally cookie, members got a painful lesson in what can happen when high demand meets limited supply. The much-hyped Rally, a raspberry-flavored spin on the Thin Mint, was always supposed to be a limited-edition cookie. But interest in it seemed to have taken Girl Scouts leadership by surprise – perhaps because of a new online-only ordering system. As demand surged, with some cookies even ending up on eBay, in some cases listed for about $40 per box, supply...