North Korea is merely the vicious dog China keeps in their backyard as a way to intimidate the US and others from walking all over them. They can leverage North Korea as a bargaining chip. While the Chinese economy and labor pool is huge they are woefully behind places like the US in technology and weapons. For example, the US military is equal to the next six or seven largest in the world combined including that of China. Hence, the reason they resort to intimidation tactics as a way to discourage countries like the US from simply walking all over them.
Coal is coal. You cant refuse to take coal from another country just because your against it. That would be like not taking an apple from a tree because you cant climb the tree or it looks funny. What's the deal? An object is an object you cant change it just on what you think about it.
China, as a leading member of the UN, should not buy coal or any other products from North Korea because of the sanctions the UN has placed on that country. It is important for UN countries to follow the agreed upon sanctions and not undermine them. Doing so weakens the sanctions and could make them ineffective.
Sanctions against North Korea cannot function effectively if China chooses to pick and choose when to enforce the boycotts and purchases materials such as coal from the Hermit Kingdom. This is not the first time that China has violated international sanctions levelled against countries accused of grave human rights abuses in order to gain a self-interested advantage. China were warned at the outset of these sanctions not to violate the international agreement which they were party to. In going back on their word, they have broken international law.
Sanctions are put in place to create a penalty or hardship for some disagreeable conduct from a foreign government. It is not proper for China to purchase coal from North Korea. The sanctions should not be overlooked or disregarded because that sends a weak message regarding the U.S. There needs to be a firm and consistent response to sanctions.
The only way it would be okay for China to purchase coal from North Korea would be if there was no other way for them to get enough coal and they desperately needed it. That does not seem to be the case. It seems as if China simply bought coal becasue it was cheap and easy.