The self-styled Islamic State (IS) has a printed list of prices to facilitate the slave trade of women and children captured in the group's territory.

Though rumors of such a document surfaced eight months ago, Zainab Bangura, UN special envoy on sexual violence in conflict, has now confirmed that she has seen the document, and that it is legitimate.

"The girls get peddled like barrels of petrol," she told Bloomberg. "One girl can be sold and bought by five or six different men. Sometimes these fighters sell the girls back to their families for thousands of dollars of ransom."

Bangura said she was personally given a copy of the document during a trip to Iraq in April.

She also provided details about how the IS flesh trade operates in Iraq and Syria, saying that leaders in the organization are allowed to take first pick of captives. Those who aren't chosen are auctioned off to wealthy locals, with the remaining captives then sold to IS fighters, Bangura said.

According to the document, it costs $165 for an IS soldier to purchase a child under nine years old. Adolescent girls sell for $124, and women over 20 cost less.

IS first started selling prisoners en masse after more than 2,000 Yazidis — a Kurdish religious minority — were captured during an offensive in northern Iraq in August 2014. Video later surfaced of IS fighters trading Yazidi women among themselves.

IS released a religious justification for its treatment of Yazidi women in the fall of 2014, saying in an article in its English-language magazine Daqib that slavery comported with Islamic law. "After capture, the Yazidi women and children were then divided according to the Sharia amongst the fighters of the Islamic State… after one-fifth of the slaves were transferred to the Islamic State's authority to be divided as khums [spoils of war]," the article read. "This large-scale enslavement of mushrik [idolator] families is probably the first since the abandonment of Sharia law."

In April, IS released nearly 200 Yazidi captives in exchange for up to $30,000 each from their families. "As ISIL regards those who have not converted as _malak yamiin _[slaves], in ISIL's eyes the money paid was the purchase price," Francesco Motta, the UN human rights chief in Iraq, told VICE News in May, using an alternate acronym for IS. "All were taken before a so-called Sharia court for the 'bill of sale' to be approved prior to their release."

The existence of the list is further evidence that IS has institutionalized the slave trade. "They have a machinery, they have a program," Bangura said. "They have a manual on how you treat these women. They have a marriage bureau, which organizes all of these 'marriages' and the sale of women. They have a price list.

"It's not an ordinary rebel group," she added. "When you dismiss them as such, then you are using the tools you are used to. This is different."