It is probably the most amusing thing in the whole story. The great part of the income North Korea gets from selling counterfeit cigarettes and pharmaceuticals like Viagra, fake $100 bills, drugs like methamphetamine and marijuana to Japan and other countries. In 2003 a merchant North Korean ship tried to deliver 150 kg of heroin to Australia. Imagine that"...
By the way, pot is legal in North Korea. Distribution, possession, and consumption of cannabis are a common deal here. Besides, it is recommended instead of tobacco because it is considered to be healthier. Marijuana grows widely here, but people are calm with it. Maybe because it isn't strictly banned in North Korea, and so people here are not so interested in it.
North Korea also sells small arms and missile parts to terrorists from so-called rouge-nations. Besides, they say, that North Korea also practices outsourcing – people from the labor camps often work in Siberia.
I was really surprised when I found out, that a lot of European and Asian companies invested in North Korea's casinos for tourists (about 1,5K tourist come here every year) and mineral mines (which are estimated to be worth $6 trillion). In 2010 these investments reached $1,4 billion.