View all things through the prism of energy, usually oil, though in this case, coal.
There is always the extra “why” question. Why was there war with Japan? Because they bombed Pearl Harbor. But why did they bomb Pearl Harbor?
Because in the Great Depression of the 1930s only one thing evolved inexorably — the US became the world’s Saudi Arabia. The big exporter of oil. FDR decided he would use oil to dictate another country’s foreign policy. Namely, Japan’s, whose invasion of Manchuria (just north of N. Korea) he wanted stopped. He cut off oil exports to Japan to stop their motorized forces. Japan faced a future of humiliation without oil, so they decided flow from Indonesia must be ensured and that meant the US fleet at Pearl must be hit.
N. Korea has coal. Geology doesn’t care about politics. That region is hydrocarbon rich. Manchuria still flows oil (the source of China’s 4.5 million barrels per day) and N. Korea has coal. All these decades of sanctions changed nothing because of coal. Coal is a number of joules per ton and that number is not determined by trading on a market in a hostile country.
Is it decent quality coal? If so it will sell regardless of sanction.