The website included online discussions from groups who "expressed their sexual interest in children" and found Mr Quinn after he posted under the pseudonym of "Mick", who said he was travelling to LA from Australia.
"Mick" told the agent he was "looking for sex with children" and while "he liked both boys and girls, his favourite were boys, aged 5-10".
"Would love to share one with you mate," Mr Quinn allegedly replied.
Mr Quinn was under the impression he was attending a sex party and a six-year-old boy would be delivered to him by a pimp for $US250 ($335), authorities allege.
It is believed two days after he landed in the United States, Mr Quinn caught an Uber to the hotel where he planned to meet "like-minded men", along with the six-year-old boy.
In the hotel room sting, the Cartoon Network was playing on the TV while Toy Story-themed cupcakes were on offer before the boy was to be introduced to the men.
Mr Quinn allegedly had $US260 (A$349) in his pocket, while a camera, film and a monopod - a one-legged support for a camera - were on his person at the time.
A sting was set up at a beachside Los Angeles hotel on May 21.
"Quinn, clearly remembering the previous discussions where he had been told the price would be $US250 to sodomise the boy for approximately an hour, he pulled out his money and gave the pimp $US260," Aaron McClellan, a special agent for US Immigration and Customs Enforcement, wrote in the criminal complaint.
"After Quinn paid the pimp, law enforcement came into the room and arrested everyone."
A US federal grand jury has charged the Australian rugby player and IVF geneticist on charges of attempted child sex trafficking and travelling with the intent to engage in illicit sexual conduct.
The secret jury handed down its decision in the US District Court in Los Angeles on June 17, but the indictment was only made public on Monday in the US.
He faces life in a US prison if convicted over charges of attempted sex trafficking of a minor.
"Michael Quinn, also known as 'Mick', aka 'southof', aka 'inhere', knowingly travelled in interstate commerce and into the US from Australia to Los Angeles County, within the Central District of California, for the purpose of engaging in illicit sexual conduct ... namely, to engage in sexual acts with a boy approximately six years of age," the grand jury indictment states.
Quinn, a Monash University-educated geneticist who worked at a leading Melbourne IVF clinic, has been held at the Metropolitan Detention Centre in downtown Los Angeles since his May 21 arrest.
The illicit sexual conduct count carries a maximum penalty of 30 years in prison.
