Sanction busting is an interesting thing because states are sovereign so if one state imposes sanctions on another, it's not illegal for the sanctioned state to do everything to ignore/bypass them, unless we consider the sanctioning state to be somehow above others/source of truth for what's legal.
It's usually illegal in the counterparty's state. If I'm in the EU and I sell weapons to Iran, Iran isn't breaking Iranian law, but I'm breaking EU law. Iran is also breaking EU law but that's irrelevant to them the same way it's irrelevant to me that I break North Korean law all the time.