North Korea is trying real hard to sell more missiles. For example. North Korea is planning to launch one of its long range missiles in the next week or so. North Korea says it is to put a satellite in orbit. Some neighboring countries believe it is another test of a ballistic missile (for delivering a nuclear warhead). Japan is threatening to shoot the North Korean missile down. But what North Korea is apparently doing is demonstrating to other countries that its missiles work, and can be had for attractive prices.
Missiles and other weapons are North Korea's major exports and sources of hard currency. This next launch, for whatever else it may accomplish, will also sell more missiles. And there are customers out there. Iran, due to lack of experience, was forced to go to North Korea for advanced missile technology. Thus the long range (about 4,000 kilometers) Taepo Dong 2 missile that is currently sitting ready for launch in North Korea, has a price tag on it. Iran is willing to pay well for missile technology, because it also offers missiles for sale.
Iran manufactures a 1,300 kilometers range missile, the Shehab-3, as well as several others with shorter ranges. Iran has offered them for sale, but has found few prospective buyers. Iran has been under an arms embargo (for most major weapons exporters) since the Islamic revolution of 1979. China and North Korea have broken the embargo, but quietly. North Korea has no such embargo problems.
Saudi Arabia, Yemen and Pakistan have also bought North Korean missiles. But if North Korea wants to sell more, they have to pitch the product. For missiles, nothing does that better than another successful launch.