Without having read the 'Sex for Sale: The Untold Story', it's difficult to say precisely. However, it's likely that the main characters are those directly affected by the sex - for - sale phenomenon. It could be the victims of human trafficking who are forced into prostitution, the sex workers trying to survive in a difficult and often dangerous environment, and the people on the periphery like the people running the brothels or the organizations trying to rescue the victims.
I don't know for sure as I haven't read the story. It could be the people involved in the sex trade, like the sellers, the buyers, or perhaps those who try to combat it.
Given the title, the main characters might be the individuals who are part of the sex - for - sale scenario. This could include prostitutes who may have their own stories to tell, the pimps who control parts of the business, and even the clients who seek sexual services. There could also be social workers or law enforcement officers who are involved in dealing with this issue as main characters in the sense that they interact with those in the sex trade.