https://www.gao.gov/assets/690/688535.pdf

This is pretty interesting. In November 2017 the GAO performed a study about how easy it was to purchase firearms illegally online. While their sample size is pretty small they found that on what they call the "surface web" they got told to kick rocks 100% of the time. When trying to illegally purchase a firearm on the "Dark Web" they were only able to make a total of 2 illegal firearms purchases.