Police said the model was sent by her agent in the United Kingdom to Milan for a photo shoot on July 10 and kidnapped by a man who posed as a photographer the following day.
After the model was freed, her kidnapper demanded €50,000 and threatened to kill the traumatised woman if she reported the incident to the police.
"Attacked, drugged, handcuffed and closed inside a suitcase, that's how a 20-year-old English model was kidnapped on July 11 in Milan to be sold to the best offer on pornography sites", on the internet, the statement said.
A Polish national, with British residence, has been arrested on suspicion of kidnapping for extortion purposes, according to reports, after the 20-year-old woman was returned to the British consulate in Milan.More news: AstraZeneca shares plunge 16 percent on lung cancer failure
The kidnappers then started blackmailing her modelling agency, demanding USD$300,000 (A$377,000) in ransom, Italian authorities said.
Herba, who is reported to now live in the West Midlands, is accused of luring the woman to Milan from Paris with a promise of a fake photoshoot.
No one took part in the online auction, so it was unclear if Herba had the necessary contacts to organise such an operation of was something of a fantasist, according to the police.
As she was released on July 17th, her captor told her: "Our rules exclude mothers".More news: Don't turn Hindustan into lynchistan, says Mallikarjun Kharge
He also allegedly told the agent that he was operating on behalf of "the Black Death group", an organization involved in illegal trafficking on the dark web, a layer of the Internet accessible only through anonymizing networks like Tor.
The woman was eventually released after six days in captivity when he kidnapper realised she had a child.
Police said she was held for a week in abandoned offices in the city and officers are now looking for accomplices.
Italian police said they are working with officials in Britain and Poland as they continue to investigate.More news: Inter try to end Man United's Ivan Perisic hopes