I know there is such a thing as selling your son into prostitution, but generally it's a much younger son. Once they get to a certain age, it would become nearly impossible to sell them off. I guess since she looks like a loli, he figures it would be easy to do? Honestly I think this whole scenario would be more believable if her mom was the one doing it instead of her dad. A woman in a ton of gambling debt in this world would get better money by selling themselves off instead of selling off their son.