It's a good question. I just thought I'd pop by and ring off a quick mention that in terms of international waters, there will always be an imposition of law. At least from my understanding (having written a lot of law stuff on the site lol).
If you're a vessel registered in or serving a nation (flag state), they impose their laws on that vessel in international waters, technically no matter the distance to said state. If you were to do a shady deal, it would be your flag state's prerogative to notice that next inspection you took or next time you returned to port or their space, at which point necessary action would be taken. They could even potentially be patrolling said international waters.
Say a Yamataian corporation flies out to international waters and buys a bunch of slaves. It would be Yamatai's prerogative (as the vessel is registered there) to apprehend the vessel and sentence its occupants under Yamataian law, even if it's still in international waters.
It's also noteworthy that vessels in international waters not registered to a flag state are considered pirates for all intents and purposes as they are technically trying to operate outside the law. I'm fairly sure they can then be apprehended by anyone, which would cover those independents trying this that you mentioned.
There are also crimes that fall under something called universal jurisdiction such as genocide, at which point it doesn't matter what flag you fly
anyone can act to bring you to justice.
EDIT:
This being said,
@Wes likely has a variant on this if it's even the case we have such waters. Especially regarding independent vessels (we likely don't want nomads to be considered pirates. Vessels without an origin aren't really a thing IRL so we can't draw from that here). However, he has mentioned plans to register all companies to nations so maybe he has other ideas for independents.