When something is a scarce resource and in high demand, it is very likely that it will be trafficked at redirection of distribution of said resource. Case in point, children, and especially boys in China.
As the article states, between 8,000 and 15,000 children (75% boys) are kidnapped and trafficked every year in China, and the authorities are not really interested in fighting this. Trafficking exists because there is a market for boys in that country. The main victims are migrant workers who travel with their children but are separated from their family. They work long hours and follow the rural custom of letting their (often unique) child play outside, on the streets of overcrowded and impoverished neighborhoods.
Who buys these stolen boys?
There seems to be three types of buyers: the childless, those who only have daughters, and those who already have a son but want more conform to the image of the traditional family. And since local authorities only report to the family planning office only births, these trafficked children are never reported. And as peasants get wealthier, they experience more pressure to pass down their wealth to their sons.
As soon as these children join local schools, their new identity is made completely legal and their biological parents have very little chance of ever finding them. This practice is relatively accepted. The parents who paid for these children take care of them, so, they do not feel they did anything wrong. Traffickers often tell them that the children come from extremely poor families or have been abandoned, so really, they are better off with a wealthier family. So, everyone turns a blind eye to this.