This advertisement -- a screengrab from an online marketplace used by ISIS fighters to barter for sex slaves -- is one of many Abdullah Shrem keeps in his phone.

Each offers vital clues -- photographs, locations -- that he hopes will help him save Yazidi girls and young women like this girl from the militants holding them captive.

Shrem was a successful businessman with trade connections to Aleppo in Syria when ISIS came and kidnapped more than 50 members of his family from Iraq's Sinjar province, a handful of the thousands of Yazidis seized there in 2014.

Tens of thousands of Yazidis fled their homes and clambered up Mount Sinjar in an attempt to escape the fighters; hundreds were massacred, while thousands of women and girls were abducted and sold into slavery.

Desperate -- and angered at what he saw as a lack of support from the international community -- he began plotting to save them himself, recruiting cigarette smugglers used to sneaking illicit produce in and out of ISIS territory to help his efforts.

"No government or experts trained us," he explains when we meet in the Kurdish region of northern Iraq. "We learned by just doing it, over the last year and a half, we gained the experience."

So far, he says, his network has freed 240 Yazidis; it hasn't been easy, or cheap -- he's almost broke, having spent his savings paying smuggling fees.

For those who venture into ISIS territory, the stakes are even higher -- a number of the smugglers have been captured and executed by ISIS while trying to track down Yazidi slaves.

But Shrem insists the risks are worth it: "Whenever I save someone, it gives me strength and it gives me faith to keep going until I have been able to save them all."

In some cases, the smugglers follow clues in the adverts or other tidbits of information they're able to gather to find the Yazidis. In others, the hostages themselves reach out and plead for help, offering key details as to their location; a province or town they've overheard mentioned, or a local landmark they've been able to spot.

Once they manage to make contact, the prisoners are told when and where to go to meet the smugglers who wait for them in a nearby car. Depending on the circumstances of the rescue, it can take days or even weeks to get safely out of ISIS territory, switching from vehicle to vehicle, and waiting in safe houses, CNN reported.

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