What were the most popular drugs on Silk Road?

In this courtroom sketch, Ross William Ulbricht is seated in court and awaits sentencing Friday, May 29, 2015 in New York. The San Francisco man who created the online drug-selling site Silk Road was sentenced to life in prison by a judge who cited six deaths that resulted from drugs bought on his website and five people he tried to have killed.
Elizabeth Williams | AP
In this courtroom sketch, Ross William Ulbricht is seated in court and awaits sentencing Friday, May 29, 2015 in New York. The San Francisco man who created the online drug-selling site Silk Road was sentenced to life in prison by a judge who cited six deaths that resulted from drugs bought on his website and five people he tried to have killed.

Just what was the most popular product sold on the Silk Road, the anonymous online marketplace whose founder has been sentenced to life in prison without parole? A study by a Carnegie Mellon researcher offers an answer.

Silk Road founder Ross Ulbricht, who went by the online moniker "Dread Pirate Roberts" after a character from "The Princess Bride," was sentenced Friday. He had faced a minimum of 20 years in prison after being found guilty on a range of charges, from drug trafficking to money laundering.

According to prosecutors, the site generated about $214 million in sales between early 2011 and late 2013, when FBI agents shut down the website, seized about $4 million in bitcoin, and arrested Ulbricht in San Francisco. In late 2011 and 2012, while the site was still active, Carnegie Mellon's Nicolas Christin gathered eight months of data that shows the 20 most popular categories on Silk Road.

Marijuana was No. 1, followed by the nonspecific category "drugs." As Christin notes in his paper, 16 of the top 20 categories are drug-related, with "soft drugs" like marijuana outselling "hard drugs" like opiates. "This presumably simply reflects market demand," he wrote.

Other categories, like "services" and "digital goods," also offered illicit purchases like hacking services and pirated content, according to the FBI. The large "books" category included titles like "Hacking for Beginners." The fourth nondrug category was erotica.

Over the eight months the Carnegie Mellon data cover, the number of unique sellers on Silk Road more than doubled. Most of the sellers—44 percent—shipped from the U.S., followed by the U.K, the Netherlands and Canada.

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The paper estimates the site's daily sales by counting customer feedback and using average item prices, coming to an average daily volume of about 7,700 bitcoin a day, or $1.22 million a month. At that rate, the site would have sold a little more than $40 million in the 34 months it was in operation—although the higher estimate by prosecutors could be accounted for by rapid growth in the last year.

That wouldn't be surprising, as average daily sales increased by 44 percent in just the months that were measured, and most customers seemed satisfied with their buying experiences—about 98 percent of all posted feedback was positive.