Read the rest of what I said before you try and flare up like you actually have something to prove. It's how it is in the Manga's world, try and use that 1.5kg brick of meat and fat between your ears instead of just writing shit off just because.
Ooh, insults. I guess I should be offended.
Here:
Happy now?
Now, let's be serious for a moment. Much like you, I only quoted a part of your comment, but I read it entirely.
You, on the other hand, seems to not have read mine.
My point is that your arguments as to why he can't adopt (missing guild card in particular) should apply at least equally - if not way more - when going to enslave someone. Even if the slavery is "special". Otherwise any kidnapper can come along and "special"-enslave a child. Which is doubly worse than adopting one if this form of slavery is also enforced magically.
Slavery doesn't -or at the very least shouldn't - instantly solve problems that bureaucracy make mildly annoying. This would create a society where slavery is not only abhorrent, but outright open for abuse - I mean way more than it is by its very nature - since the "solutions" it offers are worse by several orders of magnitude than the problem it allegedly addresses.
As for the historicity of slavery, it's irrelevant here. This is a fantasy setting that is only somewhat inspired by european medieval standards. And the slavery here is different than it was then. (Or still is in some places of our own modern world.) Moreover, I don't necessarily have a problem with an anime character using slaves in some settings. I
do have a problem in the case of isekai MCs, because they should know better. At the very least, like Sora in a previous chapter, they should be reticent unless it's a last resort. Or a psychopath.
See the Shield Hero for an example where it mostly works. Trapped in a world where he knows nobody, framed by the princess of the country that summoned him and despised by the other isekai'ed heroes - not people he knew - and told to fight for his life with 0 attack power - though he had a partial workaround for that.
That was one cornered individual whose last resort was to buy a slave.
In comparison, Sora has a problem with the rulers of the country that summoned him, but the comparison stops here. He has already built trusting relationships with a few individuals, has civil relations with most others and is living an overall comfortable life. He isn't anywhere close to needing slavery unless the plot forces it on him in the worst way possible. Like telling him that he can't take care of a child he just saved unless he literally (re-)enslaves her.
Now, I hope you'll pardon me for not concluding with some braindead insult, as seems customary in your culture.