Italian police say the unidentified model was drugged and kidnapped in the Italian city before she was kept as prisoner for a week.
Herba freed her on July 17 and accompanied her to the British consulate in Milan.
Following the attack, the woman - who can not be named for legal reasons - was put in a duffel bag and taken around 90 miles to Borgial, an isolated village near Turin, before being released on July 17, police say.
Police have arrested a 30-year-old Polish man - believed to be resident in the United Kingdom - in connection with the kidnapping.
Police say they are still unclear whether the kidnappers were running an online scam to con "buyers" out of money or whether they actually meant to sell her on.
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.
The websites for the auctions included a description of the victim and a starting price but police said it was still unclear whether the suspect had made them up.
"You have a two-year-old child and our rules exclude mothers", the kidnapper allegedly told her according to the Italian report.
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Prosecutors in Italy said Herba was linked to the Black Death criminal organisation that works on the dark web.
It added that she had been let go after one of its men made "a clear and solid stance in your case".
He has been charged with kidnapping for the purposes of extortion.
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 police investigation into the network is ongoing.
Police said they were not sure if the group actually existed.
The Foreign Office confirmed it was providing assistance to the woman in Italy and is also cooperating with local authorities.