Sandra, a teenage girl who wears her curly brown hair tied back in braids, awaited the instant message on her mobile phone. The instructions were matter of fact: Wear makeup and a short skirt. If possible, don a crop top.Like other girls in her neighborhood outside Medellín, Colombia, Sandra said she didn’t always have food for dinner, let alone trendy clothes and electronics. But a friend tipped her off to a sure-fire way to make money fast. This amiguita, she said, told her about the plentiful...