The reports I've read suggest that the Norks are only able to make enough material for 2-5 nukes a year. So they've tested at least a year's supply. How long do they last before you have to reprocess the uranium due to too much decay? What's the shelf-life of a nuke?
Anyway they've not got many, so may not wish to sell them. Plus there's a good chance selling one would be traceable. The retaliation after that might be quite serious. The regime might struggle to survive without imports of fuel from China - that would almost certainly tip China over the edge into becoming an enemy. I doubt the regime can survive that.
Also they're under very serious sanctions now, so ships can't just leave North Korea - which means the nuke would almost certainly have to be smuggled via China. Which might also upset them somewhat. It's possible though, they may be mad enough to sell or use them.
But selling the technology is nothing new. I don't remember the exact details of who helped who now, but look up the AQ Khan nuclear network. My recollection is that China helped Pakistan's nuclear program, as a counter-balance to India getting them in the 70s. Pakistan was a bit light on missile technology though, so they were helping various people in exchange for missile technology. I think that was principally Iran and North Korea, but possibly also Libya (not that their nuke program seems to have been all that serious). I don't remember if it was Pakistan or Israel that was cooperating with the South African nuke program.
Iraq had a mix of Russian, Chinese and self-modified Scuds. I don't think they were using the North Korean tech. Again I think Iran and the Norks were cooperating there, so Iran would not have been best pleased if North Korea were helping their main enemy as well.
I don't remember reading anything about Syria's program, so don't know who they were talking to. The AQ Khan network was broken up over ten years ago, and the Syrian project was more recent. I wonder if they were using Saddam's old scientists - his nuke program doesn't seem to have got all that far, and Israel bombed the French reactor in the 80s - but they must have still had the info. Or they could have been talking to North Korea.
Tis all very convoluted.