It is almost 01:00 on the Iraqi-Turkish border and the guards are preparing to close the gates for the night.

As passengers hurry to get on the last buses heading into Turkey, a Yazidi family are standing silently, eyes fixed on the crossing point.

Suddenly, a woman and four children appear from the Turkish side. The family rush to greet them and the whole group dissolves into tears.

As they hug each other they keep looking into each other's eyes, unable to believe they are finally together again.

Khatoon, 35, and her children - aged between four and 10 - were captured by ISIS militants who stormed their village in Iraq's Sinjar region in August 2014.

They were taken to Raqqa, the de facto capital of the caliphate declared by ISIS two months earlier.
Khatoon looks exhausted and barely able to stand. "It was horrible," she says. "They didn't give us enough food or water, or let us wash. Sometimes they beat us."
Escape
Khatoon and her children owe their freedom to an Iraqi businessman named Abdullah, who used to buy agricultural goods from Syria but now buys people held captive by ISIS.

After reuniting Khatoon with her family, Abdullah takes us to his modest house and introduces us to his 22-year-old niece, Marwa.

Like Khatoon, Marwa and 55 of her relatives were seized by ISIS fighters in Sinjar a year ago.

Two months later, Marwa managed to contact her uncle and tell him she was being held in a house in Raqqa,

"I told her: 'They can't understand Kurdish, so listen carefully. If you can find a way to get out of that house, I'll try to find someone to bring you back," Abdullah says.

One night Marwa managed to steal the front-door key and make her escape.

"I waved down a taxi," she says. "The driver asked me where I was going. At first he said he was scared, because if he was seen with me, ISIS would kill both of us.

"But in the end he agreed to take me to another part of town, where there were good people who could help me."

The next day she tried to call her uncle but her ISIS captor had found out where she was staying. He demanded that the family who had taken her in either to return her, or to buy her from him for $7,500 (£4,810).

He also contacted her uncle."I told him: 'OK, give me some time and I'll send the money to you. But don't touch my niece,'" says Abdullah.

He began calling old business contacts in Syria and eventually secured Marwa's release.
Setting a price

Over the past year, Abdullah has built up a network of contacts and smugglers across Syria, Turkey and Iraq and managed to free more than 300 mainly Yazidi women and children from ISIS captivity.

He has found it costs between $6,000 (£3,850) and $35,000 (£22,450) to buy someone back from ISIS.
For young girls the asking price is even higher, and even babies are not exempt.

"Once a family had to pay $6,000 for 30-day-old baby," says Abdullah For many families finding that amount of money is almost impossible.

Khatoon and her children were freed after her father-in-law, Mardan, paid $35,000.

"I sold everything I had," he told the BBC. "I had to go door-to-door borrowing money. Now I have to pay it all back, but I'm penniless and 17 members of my family are still being held by ISIS."
Not all operations to buy back captives are successful.

Earlier this year, Mardan raised $35,000 to secure the release of his other daughter-in-law and her two children.

However, the Kurdish smuggler who was acting as go-between was killed and the $17,500 that he was supposed to be passing on to the ISIS captors was lost.

They are now demanding that Mardan sends another $10,000 if he wants to see his daughter-in-law and grandchildren again.

'Only option is to trade'

Although Abdullah says much of the money involved in ransom operations goes to the people smugglers rather than ISIS, he knows his activities are helping to fill the group's coffers. But he sees no other way to reunite the captured Yazidis with their families.

"For ISIS women and girls are nothing more than goods and our only option is to trade them like you would trade goods and products over the border," he explains.

The most difficult part of these operations, he says, is confirming that contacts on the ISIS side are genuine.
Over the past year, four of the 23 smugglers working with Abdullah have been killed by ISIS.

"Sometimes they call and tell us to come and pick up some Yazidis," he says. "But when we send someone into ISIS territory, they are captured and killed."

The Iraqi government and the Iraqi Kurdish authorities have now set up an office to co-ordinate efforts to secure the return of Yazidi captives, But it is clear they are struggling to cope.

"There are still too many people being held by ISIS and we haven't got enough money to pay smugglers to bring all of them back," says Noori Osman Abdulrahman, the Kurdistan Regional Government's co-ordinator for Yazidi affairs.

For many families, going it alone with Abdullah's help is the only option.

However much ransom is paid, it is clear that for both the captives and those who have been waiting for them that it is the emotional cost of their ordeal that will take the highest toll.

Marwa, is still struggling to come to terms with what happened.

"When I get upset, I get panic attacks and can't breathe," she says. "I get flashbacks about what happened to me and all the other girls."

Despite Abdullah's efforts, so far she is the only one from her family to have escaped.