Lukasz Herba was arrested July 17th outside the British consulate in Milan while accompanying the woman, said Lorenzo Bucossi, the head of the Milan Police Mobile Command unit.
The model, who has not been named, was assaulted, drugged, handcuffed and stuck in a travel bag, police said in a statement.
The woman was eventually released after six days in captivity when he kidnapper realised she had a child.
Police say the model was freed after discovering she had a young son, which broke the vile group's rules to exclude mums.
He said the woman was to be sold to the highest bidder on deep web pornographic sites and threatened to kill her if police were notified.
The unidentified 20-year-old woman was allegedly captured by Lukasz Herba, 30, a Polish man posing as a photographer on July 11, local news outlet Corriere Della Sera reported.
During her detention, Herba had demanded the agent pay $300,000 to avoid the model being sold online though no ransom appears to have been paid, police said.
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Officers went through Herba's phone and computer and said they found photos of the woman and material stating she was for sale, while hair from both the model and Herba was found in the boot of his auto.
Prosecutors described the kidnapping as a "very well-organised seizure" and added he was contacted over the internet two days later with requests to "buy" her for sexual purposes.
Italian investigators have since established that the suspected kidnapper had already organized several online auctions for the sale of abducted girls, referring to them as "prey".
Police found a small bedroom at the property, but it is unclear whether it was used by the model or the kidnapper.
Italian police said they are working with officials in Britain and Poland as they continue to investigate.
The Foreign Office says it is providing consular support to a British woman.