what do they even have to sell?
I remember on some VICE article, they were talking about how North Korea funds itself by an unofficial means.
Their largest source of income comes from selling human labor to Russia. They also sell drugs such as methamphetamine, cocaine, and marijuana through unknown channels, probably the same channels they sell people through.
There's also the tourism sector, which was jointly created by NK and China. Lots of tourism to NK to see the "isolated" state and see what it's really like, mostly chinese citizens, but there are also those who use China as a means to document what's going on inside NK. That's how Vice and a dozen other internet news websites have gotten journalists inside NK.
Oh yeah and you can buy NK merchandise off their official website. EDIT: found it.
http://www.korea-dpr.com/