In 2005, Ms. Brainard-Barnes was a 22-year-old single mother who had difficulty holding down a steady job. She never could afford her own place, staying with her sister for long stretches and occasionally with a boyfriend. Modeling, she thought, could lead to a music career. As soon as she enrolled on a site called ModelMayhem.com, she received dozens of e-mails from places that purported to be modeling agencies but that, after a few clicks, turned out to mean nude modeling and sometimes working as an escort. She wasn’t thrown by seeing this. What did surprise her was the money.