Over the last decade, North Korea has exported about $800 million worth of weapons, (mainly to Iran, Syria and Myanmar). That's puny by international standards, where total arms in the last decade were over $500 billion. But for North Korea, with a population of 22 million, and a GDP of about $26 billion, $80 million of arms sales a year is a big deal (as in over three percent of GDP, and a major source of hard currency for buying imports.)
But with a growing number of international sanctions on North Korea (for breaking agreements, and developing ballistic missiles and nuclear weapons), it's getting increasingly difficult to deliver these weapons. In response, North Korea has adopted many tricks to hide the movement of these weapons. The easiest one is to move the weapons by rail, through China or Russia, to a port in those countries. From there, a Chinese or Russian cargo ship (or one from another country) can deliver the goods. The ships are chartered by an off shore company that has no publically known connection to North Korea. False papers can be obtained for the cargo, and so on.
The U.S. is wise to most of these subterfuges, and, if the resources (intel and spy satellites) are available, the North Korea shipments can be tracked. But the biggest obstacle the North Koreans face is the growing Chinese and Russian anger at North Koreans nuclear weapons activities. The Chinese and Russians could quietly forbid anymore North Korea weapons shipments, and that would force the North Korea to fly the goods (at least the stuff that would fit), or take their chances using ships going to a friendly port, and trying to secretly move the goods to another ship for the final journey.
North Korea knows that it is in a tight situation here, and has openly threatened war if any of its ships are interfered with. Given the sorry state of North Korea's armed forces, this may be an empty threat. But no one really wants to play chicken with the North Koreans.