What did people buy on Silk Road?

Last month (29 May 2015) the founder of Silk Road, the online black market, was given a life sentence.
But why was the site so popular? The short answer is that it made the purchase of illegal drugs triaghtforward and secure.

According to prosecutors, the site generated about $214 million in sales between early 2011 and late 2013. 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 purchased on Silk Road.