Hmm... why would Ukraine be involved in something like this, something certain to bring bad press?
Are they desperate for extra revenue, so that even while they're in the midst of a low-grade conflict, which at any time could escalate... they sell off parts of their arsenal?
Are they doing it even while expecting US diplomatic cover within the UN?
Or more likely they are operating as a fence, a funnel for weapons into a US sponsored proxy war. South Sudan has been a US project since the beginning. Washington has largely turned against Salva Kiir and these weapons were going to the Riek Machar faction opposing him. The fact that the arms dealer was Polish is also noteworthy considering Warsaw's cozying up to Washington in recent years.
Kiev, seat of a volatile country with a government installed by and dependent upon the West would not risk the ire of Washington. If the US was really concerned about calming the situation in South Sudan there would have been a diplomatic row over this. But instead, the story has gotten very little play.
It's hard not to be suspicious. This interesting side story is likely part of a larger and much more complex web of Western machinations and proxy wars within Africa and Eurasia. The money trail and operations find their nexus in Langley, the Pentagon, Wall Street and elements within the White House.