Selling the Girl Next Door takes viewers into the world of underage American girls caught up in the violent sex trade. Thousands of girls under the age of 18 are ensnared into lives of prostitution annually, according to the National Center for Missing & Exploited Children. Many are runaways or “throwaways” trapped in “the oldest profession” by pimps who sell them using modern sales and marketing techniques, including the online classified website Backpage.com.