An illegal market of children has developed in China, in which babies are being openly sold online. Police say many of the victims are from the estimated 20,000 children abducted each year - a crime with a devastating impact on separated children and parents.

It was a few days before the Chinese New Year in February 2007 and Xiao Chaohua was working at his small clothes shop in an industrial zone of Huizhou, a city in China's Pearl River delta, not far from Hong Kong.

Xiao's five-year-old son, Xiaosong, was in a playful mood. A couple of hours earlier, they had visited a nearby beach where they had made sandcastles and paddled in the surf. "He was so happy that day," says his father. "I took lots of pictures of him on my phone."

At about seven in the evening, as dusk was falling, Xiaosong asked his father for some money to buy his favourite snack - the sweet milk from a grocery shop just around the corner. His 10-year-old sister, Xiao Lu, would accompany him. Xiao thought nothing of it and gave Xiaosong some small change.

Xiao realised something was wrong when his daughter returned alone. The two children had split up after Lu had gone to speak to a friend.

He immediately began searching. At first he checked the grocery store, then the internet cafe, but found nothing. He called the police and an officer arrived.

Xiao says the policeman said there was no point in looking as his son had probably been snatched and already taken to another city. But Xiao was undeterred. A few hours later, he found a migrant worker who he had previously seen playing with Xiaosong at a games arcade. The man was taken to the police station for questioning but was released as there was no evidence against him. He then vanished.

During the first week, Xiao paid for a missing person advertisement on local TV. It cost him $8,000. There was no response. So he kept on searching. Days became weeks, weeks became months, and months turned into years. "I had no plans," he says. "I just went crazy. I just went wherever I could find people, wherever I could find crowds."

For the first year, he rode his motorbike across Guangdong province - China's manufacturing heartlands. He would put up missing posters in bus stations, train stations and shopping malls, offering a reward for any information.

Eventually he sold his shop and with the proceeds bought a van. His wife got a job at a shoe factory and their daughter was sent back to their home town in Jiangxi province to be looked after by her grandparents.

And then Xiao began an extraordinary journey across China. He travelled from the edges of the Tibetan plateau to the country's mega-cities, to small villages down dusty roads. His journey has taken him to places he could never have imagined.

Thousands of Chinese parents experience Xiao's agony every year. The Chinese government provides no figures, but the US State Department has estimated that 20,000 children are abducted annually, or 400 a week.

Chinese state media have suggested the true figure could even be 200,000 per year, though the police reject this higher estimate.

A baby boy can sell for up to $16,000, it is reported, double the price for a girl. There is a traditional preference for boys in Chinese culture as they carry on the family name and provide financial support for elderly parents.

Once abducted, children are most often sold for adoption but some are forced to work as beggars for criminal gangs. The vast majority of those abducted are simply lost forever.

Child trafficking first received wide publicity in China 12 years ago, when police in Guangxi province discovered 28 babies in the back of a bus. They had been drugged to keep them quiet and then stuffed inside nylon bags, where one died from suffocation. The traffickers were caught and the leaders sentenced to death.

But just as the authorities have stepped up the fight, in recent years, child traffickers have become more sophisticated. A lot of activity now takes place online.

According to the state-run news agency, Xinhua, four criminal gangs smashed by police in February last year were offering "unofficial adoption" as a front to sell stolen babies via websites and online chat forums. The operation led to 1,094 arrests and the rescue of 382 babies.

There are still troubling signs of a market in children on the internet today.

BBC’s Martin Patience spotted one post online offered to sell a “healthy eight-month-old baby girl” for a “child-raising fee” of $32,000. It went on: “Do not disturb if not serious.”

The BBC secretly filmed undercover at a hospital - where a doctor said abandoned babies were frequently sold into adoption.

"There are too many babies born outside of the family planning laws," she said. "As long the families make a deal and it's done right after birth - nobody needs to know."

Under the country's strict family planning rules many families are limited to one child, and will face stiff fines if they have more.

There are also families that have a daughter but would prefer a son.

Children who have been sold by their parents can nurse a lifelong sense of rejection.

Xiao, now 39, is polite and courteous. But there is a distance when you talk to him. His eyes are lifeless - a sign of the burden he is carrying.

At present, Xiao works for an anti-trafficking organisation, the Suishou Public Welfare Fund, which helps parents find missing children by posting pictures of child beggars online.

While Chinese courts already deal firmly with traffickers, Xiao wants tougher penalties for people who buy children.

Currently they can face up to three years in prison - but experts say that in reality most buyers are never charged.

Until the law is changed, Xiao believes the black market in children will continue to thrive.

He has already travelled tens of thousands of kilometres, but says his journey will only end when he dies or he finds his son.