It is little wonder that . . .

The South Korean President is very concerned with the tone, quality and amity of the US and N. Korean peace discussions:

https://www.aljazeera.com/news/2018/07/accuses-north-korea-illegally-smuggling-oil-products-180712161217886.html

If things were going well, he would have no need to voice his concerns.

As for the oil, China would have quietly shipped whatever N. Korea. 

It is obvious that N. Korea thought it could test the goodwill of the US by being open about the oil needs it had and to be honest about it and get it on the open market.

The Trump revealed themselves as righteous and utterly crude and unconcerning of the welfare of its opponent.