The request posted last year on the website alibaba.com was surprisingly straightforward for such a dubious proposition.
A buyer who said he was representing Iranian interests was looking for yellowcake uranium, also known as Uranium 308, which is made from raw uranium and can be further processed and used in the manufacture of nuclear fuel and weapons.
On Wednesday, the authorities say, the man who answered the request and agreed to sell 1,000 tons of the material arrived at Kennedy International Airport, bringing what he said were samples hidden in the soles of shoes tucked away in his luggage.
But the sale was a setup, and the buyer was actually an undercover agent for the Department of Homeland Security.
The man, Patrick Campbell, 33, from Sierra Leone, was arrested on charges of knowingly brokering goods destined for Iran, according to a criminal complaint filed in a US District Court in Fort Lauderdale, Florida. Campbell was being held in New York City on Friday, pending a hearing.
A law enforcement official said the material in Campbell's shoes was not enriched uranium.
But it was unclear whether more tests had been done to determine exactly what Campbell had brought into the United States. The undercover Homeland Security agent was sceptical of Campbell's ability to deliver, according to court documents.
Their communications began soon after the agent posted an advertisement on alibaba.com in 2012. The agent claimed to be "an American broker representing the interests of individuals in Iran who were seeking to purchase Uranium 308", according to the criminal complaint.
Campbell replied to the ad, saying he was affiliated with a mining company that sold precious materials, including chromium, uranium and gold.
The agent told Campbell he was seeking to have the uranium "delivered to a country in the Middle East to be disguised with other types of ore and which would yield 1,000 tons of purified element", the complaint read.
Campbell claimed to have previously sold uranium "to agencies in China and Ecuador", according to the complaint.
Campbell wanted assistance in getting a visa to travel to the United States and money for the trip. The agent replied that, as a broker, he could not help with the visa and "could not spend $30,000 on a contract that had not yet been executed".
By this month, Campbell had found a way to secure a visa.
The two planned to meet in Miami, and Campbell sent a contract to the agent for the sale of 1,000 tons of uranium.
On Monday, Campbell told the agent he would be arriving in the United States in two days and "would be bringing samples of the U308 and that he had consulted a friend at the airport about how to get samples on the plane", the complaint read.
When he arrived at Kennedy, where he was going to transfer to a flight to Miami, agents were waiting for him.