@Freshguac This pretty closely resembles the way slavery worked in Japan up until the very late 1500s, where the only instance of anyone particularly important seeming particularly outraged by slavery that stands out enough to mention was Hideyoshi getting mad at foreigners for buying Japanese folks... from other Japanese folks. Didn't seem to mind his people selling each other so much, just that they were being sold off
outside the country. Gotta keep that labor force local, I guess?
This MC in particular feels disconnected from the world he's in as being "reality," so honestly he probably just looks at this kind of thing as a look back into the history books or something. We could probably spend an obnoxiously long time discussing why other MCs are as okay with it as they are in a more general sense because most of these isekai are Japanese stories, but I really don't feel like getting into that. Best I can do is a "Good luck with that" to your "I'm still waiting for an isekai where the MC actually detests slavery because so far none have done that."