Tripla's parents sold her for £170 to a man who had come looking for a wife. He took her away with him, hundreds of miles across India, to the villages outside Delhi. It was the last time she would see her home. For six months, she lived with him in the village, although there was never any formal marriage. Then, two weeks ago, her husband, Ajmer Singh, ordered her to sleep with his brother, who could not find a wife. When Tripla refused, he took her into the fields and beheaded her with a sickle.

When Rishi Kant, an Indian human rights campaigner, tracked down Tripla's parents in the state of Jharkhand and told them the news, her mother broke down in tears. "But what could we do?" she asked him. "We are facing so much poverty we had no choice but to sell her."

Tripla was a victim of the common practice in India of aborting baby girls because parents only want boys. Although she was born and lived into early adulthood, it was the abortions that caused her death. In the villages of Haryana, just outside Delhi, abortions of baby girls have become so common that the shortage of women is severe. Unable to find wives locally, the men have resorted to buying women from the poorer parts of India. Just 25 miles from the glitzy new shopping malls and apartment complexes of Delhi is a slave market for women.

Last week, an Indian doctor became the first to be jailed for telling a woman the sex of her unborn baby. India is trying to stamp out the practice of female foeticide. But in the villages of Haryana, the damage has already been done. Indian parents want boys because girls are seen as a heavy financial burden: the parents have to provide an expensive dowry for their weddings, while sons will bring money into the family when they marry, and have better job prospects.

But in Haryana, so many female foetuses have been aborted that there aren't enough women for the men to marry. The result is a thriving market in women, known in local slang as baros, who have been bought from poorer parts of India. Anyone in the villages can tell you the going rates. The price ranges from 3,000 rupees (£40) to 30,000 rupees for a particularly beautiful woman. Skin colour and age are important pricing criteria. So is whether the woman is a virgin.

When the police arrested Tripla's husband, he could not provide a marriage certificate. Generally, there is no real marriage. The women are sexual "brides" only. Sometimes, brothers who cannot afford more share one woman between them. Often, men who think they have got a good deal on a particularly beautiful bride will sell her at a profit.

Munnia was sold when she was only 17. Considered particularly beautiful, she was resold three times in the space of a few weeks. Like Tripla, she came from Jharkhand, but she was lucky: she escaped. Today she is in a government shelter for women. As she tells her story, she breaks down in tears several times.

"My father sold me to a man called Dharma," she says. "I don't know if he paid for me or not. I came to Delhi with my mother on the train, and then Dharma took me to his village. He used to beat me very badly. He used to hit me until I allowed him to sleep with me. Usually it went on for half an hour."

She was with Dharma just 20 days before he sold her. Her route criss-crossed northern India: Dharam took her to his home in Rajasthan, before selling her to a man in Haryana. "He told me: 'I have sold you to a man for 30,000 rupees'," she says. "But when we got there I realised that man wanted to sell me on as well. Then I ran away."

She found a social worker who helped her escape. In that she was fortunate: few of the women who run away from the villages where she was make it out alive. Government medical tests found she had been raped by two men. She was only 17 at the time, and the age of consent in India is 18.

"My father told me Dharma would marry me, but the marriage never took place," she says, blinking in the sun. She is deeply traumatised by her experiences; all the time she speaks, her hands play nervously with her shawl. When we ask if she wants to go home, she says: "I don't know anything. I have no will and no hope in this world."

She is the lucky one, all the same. In the villages she escaped from, hundreds of women are trapped in similar slave marriages. The village of Ghasera is a world away from nearby Delhi. It is still walled, like a fortress from centuries ago, and you enter through a narrow gateway. The roads are dirt and the houses ramshackle huts: It is hard to believe you're just an hour and a half's drive from the bright new India that is being courted as an ally by the US and attracting investors from across the world. More than 100 brides have been imported to this village alone, according to locals.

The people are hostile and crowd round strangers suspiciously. Even the police don't risk coming in to these villages unarmed. Villagers have attacked police who tried to rescue the brides, and set their cars on fire.

Anwari Katun was sold for £130 and brought here from Jharkhand. The house she is living in now is thick with flies, so many they make patterns in the air as they swarm. A small girl is asleep in the corner, flies crawling over her face.

Ms Katun wants to tell her story, but the villagers crowd into her house and stand by menacingly as she tries to speak. Her fear is evident as they stand by. Most prominent is an old woman who moves forward threateningly when Ms Katun says she is not happy. Cowed by the crowd she says: "I accept what happened to me. I'm not happy but I accept it. This is a woman's life. The only thing I want is that this doesn't happen to my sisters, that they never get sold like this."

With that, she sits in silence. Desperation is written on her face, but she is afraid to say any more with the villagers crowding around. Once they are here, with no family and no friends the women are helpless.

Rishi Kant has spent the past four years rescuing women like Ms Katun. A jovial man in designer sunglasses, he once spent four nights in Delhi's notorious Tihar jail when police carried out mass arrests of protesters at a human rights rally. His organisation, Shkati Vahini, has rescued more than 150 trafficked women. But he says he can do nothing for Ms Katun at the moment. The government women's shelter in Haryana state has places for only 25 women, and it is full. When there is no space, he can do nothing: there is nowhere else safe for the women to go. As soon as a place opens up, he says, he will go back for Ms Katun.

To get the women out of the villages, he has to enlist the help of the police. In villages such as Ghasera, the police only raid in heavy numbers, and only in the middle of the night, when they can take the villagers by surprise. Otherwise, the heavily armed villagers will resist by force. But the police are co-operative, and do get the women out. Then the long process of tracking down their parents, and trying to get them home, if possible, begins.

Getting the women out of the villages is often not easy. Recently, Mr Kant found a trafficked woman who convinced him that the man who had brought her to Haryana was running a business, and had several more women. He and the police waited in the hope the woman could lead them to the trafficker. But when they got back the next day, it appeared he had become suspicious. The woman had disappeared. Mr Kant believes she was probably sold to another part of India. He hasn't found any trace of her.

Many of the trafficked women in the villages are minors. Shabila came to Ghasera from Assam, a thousand miles away. She says she is 25, but she doesn't look a day over 15. One of the women in the government shelter, Havari, looks the same age. She is highly disturbed and talks at one moment of having had a baby, then denies it the next. She has hacked off all her hair. There is no psychiatric counselling for the women.

One of the women in Ghasera told us her sister had been sold to the village along with her, then kidnapped from it and exported to Oman. She was desperate for help to get her out.

Some of the trafficked women become traffickers themselves. Maryam, who was sold here from her native Maharashtra in 1985, has just arranged the sale of another woman, Roxana, to the village for 10,000 rupees. Although Ghasera is poor, it is better off than many of the remote villages the women come from. With their contacts there, the trafficked women can easily entice others to come voluntarily. But once they come, there is no way out. Some of the women become reconciled to their lives. Afsana speaks openly in front of her husband of her unhappiness over the years here: she is not afraid of him. Although there was no formal marriage, they have stayed together.

"I never thought I would come here. I never even thought about where Haryana was," she says. "There are several girls who do not want to stay, but what can they do? They are in a helpless situation."

Her husband, Dawood, could not get a wife locally because he has a damaged eye. He travelled to Bihar and saw several women before choosing Afsana. He paid £40. He complains that there aren't enough women in Haryana, but he does not see the link between aborting female foetuses and the shortage of women.

In Asouti, a village a short drive away, you can find the reason behind all the suffering of the slave brides of Haryana. Lakhmi Devi had five abortions, each because the child she was carrying was a girl. She had already given birth to four daughters.

She is still tortured by guilt over the abortions. "It is better for a mother to die than to kill her daughters," she says. "I was under immense pressure from my husband's family to provide him with a son. My mother-in-law even demanded I get another woman to sleep with my husband to give him a son." Eventually, she gave birth to a boy, Praveen, and her agony was over.

A recent study by Indian and Canadian researchers found 500,000 girls are aborted every year in India. Today Haryana has only 861 women for every 1,000 men. Strict laws have been put in place to prevent the practice. Abortion is legal in India but testing the gender of a foetus is not. Anil Singh, a Haryana doctor, was sentenced last week to two years in prison for telling a woman she was carrying a girl and offering an abortion.

But still, the abortions go on. To get round the police, doctors have started using codes to tell the people the sex of their baby: if the ultrasound report is written in blue ink, it's a boy; if it's in red ink, it's a girl. If the report is delivered on Monday, it's a boy, if it's Friday, it's a girl.

Meanwhile the trafficked women keep coming, from across India, to fill the places of the unborn women.