Many of these girls are sold into lives of perpetual rape in Chinese brothels, from which they will never return.
The "lucky" ones are forced into marriage with strange men, where they will be raped and forced to bear unwanted children.
But who are these men, and why are they buying so many foreign girls?
In one case that received international attention this month, a Chinese man was caught with a trafficked girl, supposedly his "wife", who could not speak his language.
He was variously reported as 35 or 40 years old; she was only 12, and pregnant.
In my experience, this is an unusual case. The girls sold as brides are generally in their mid-teens, while the younger girls are more often sold into prostitution.
In the late 1970s, the Chinese government formulated their notorious "one-child" policy to limit population growth.
Under the new laws, many couples were only permitted a single child. Those who violated the laws faced heavy fines, and enforced abortion or sterilization.
In Chinese culture, as in many parts of Asia, a boy child is valued more highly than a girl.
A boy will continue the family line and take care of his parents in their old age. A girl, on the other hand, will essentially leave her birth family when she marries.
To safeguard their own futures, Chinese couples wanted boys. Tens of millions of baby girls were killed, abandoned or illegally aborted.
China now faces several very severe social problems as result of the "one-child" policy.
There's an rapidly ageing population, with fewer children being born.
There's the "little emperor" problem, with a single child being spoiled by the undivided attention of the entire family.
There's the "4-2-1" problem, with one adult child needing to provide for two parents and four grandparents.
There are illegal children who must survive without ever having access to education, health care or legal protection.
One problem, however, has spread far beyond national borders: China has developed the world's most serious gender imbalance.
Now that the boys have grown into men, there simply aren't enough women for them to marry.
According to 2014 official figures, there are 33 million more men than women in China, and the reality may be even worse.
Without women, these men have no way of continuing their family lines. Their demand feeds a monstrous industry in human lives.
Pregnant women, babies, and children are all at risk of being kidnapped - both within China and from neighbouring countries, where borders are porous and border areas are often populated by poor, and poorly protected, minority groups.
The most valuable victims, however, are teenaged girls - and especially virgins - who are physically ready for having sex and bearing children, but have not yet done so.
Counter-intuitively, the men buying trafficked girls are not wealthy. Chinese men with wealth and social status are able to marry Chinese women.
The men buying trafficked girls are those who are otherwise unlikely to find a wife. They may be poor, physically unattractive, or disabled.
A trafficked girl can be bought for just a few thousand dollars - significantly less than the cost of wedding gifts to marry a Chinese woman.
Often, there will be a ceremony to give an air of legitimacy to the "marriage", despite the fact that the trafficked girl has no legal status in China, and is unable to communicate.
She will then be kept prisoner by her "husband", who will force her to perform housework, have sex, and bear one or more children.
The majority of these girls will never see their homes or families again.
The men buying these girls can also be considered victims of circumstances beyond their control - but this cannot excuse treating other human beings as objects to be bought, sold and abused.
While the "one-child" policy has now been relaxed and is to be relaxed further, this demand for young women will continue for at least another generation.
Chinese birth rates are still incredibly imbalanced - in 2014, according to official statistics, there were 115.88 boys born for every 100 girls.
Those found guilty of trafficking can be heavily penalised - but the Chinese government is unable to provide a better solution for the men who cannot find Chinese brides.
While there's little we can do to alter China's gender imbalance, we can help protect the girls most vulnerable to the very real horrors of human trafficking.
Over a thousand people have now contributed $36,482 to help us raise awareness of this monstrous issue, and to launch a human trafficking prevention program in high-risk villages of northern Vietnam.
If you haven't yet contributed, there's only one week remaining.
Right now, you can help protect girls in danger, help yourself to some great rewards, and get a very special sneak peek at our feature documentary, 'Sisters For Sale'.
Make a difference now at www.sistersforsale.com