@Tamerlane What you say is certainly something worth thinking about, though as others have pointed out, there is the issue of slavery simply being a part of this world's system, meaning it would be even harder to eradicate than in ours.
However even doing away with the particular case of this manga, I can't fully agree with you.
You're right that the request for slaves will incentive slave merchants to get more slaves, and as such it's reasonable to say that if you want to cripple the system as a whole you should not take any part in it.
You're also correct in saying that buying and then freeing them is a way to feel good about yourself, even though it may, in the end, be bad if you want slavery to end.
Still, that can only be true when you go about it in a cold, detached and purely logical approach (and even in this case it may, for other reasons, not prove to be the best choice, but I'll expand on this later).
Yes, if you look at it from the outside, especially as it is for us, who are just people reading a silly story, he's doing something that is potentially positive for the system of slavery as a whole.
At the same time, we are talking about people, human, or at least human-like beings.
Not buying a poor little girl might in some undefined future prevent the commerce of further slaves - yet that girl will remain a slave, and possibly be bought by someone who will torture and kill her.
Is that right? Is that good?
It could be the right approach to end slavery, and as such it might prove "more just" in the long run, but could you in good faith say that is the right choice as you look that girl in the eyes?
I'm not saying you're wrong per se, and indeed what you suggest has merit, but it is still a serious dilemma.
This is something that can be applied to many other real-world issues.
Supposing for example that capitalism is, as many believe, generally positive for the world, is it right to diminish, or at least not augment, aid aimed at less fortunate people?
Or on the other hand, supposing socialism is, as some think, a system that is more humane and would prove much better, would it be right to cause untold suffering through a revolution?
Or at least withold more rapid progress through a slow reform, to then reach that supposed superior ideal?
Should we push for multiculturalism at any cost and take joy in our shared brotherhood and connections, even though it might result in serious backlash and even lead to armed conflict?
Or should we enforce ethnic separation so that everyone can live in a homogenous society, even if it means forcefully removing foreign people or worse?
Maybe every problem in the world can be resolved through technology, so all our efforts should go there, with litte regard for the people working jobs which will be taken over by automation. After all one day everyone will be better off if we develop a perfect virtual reality and do away with scarcity, no?
And all the same, wouldn't we be much happier by reverting to our ancient past, when we lived in harmony with nature as hunter-gatherers, wandering through forests and mountains, freely running through sunlit meadows, each day nobly fighting off Mother Nature's other children, as true men. Maybe to do that most of the current population ought to die, and those who'll survive will have to re-learn long-forgotten abilities, but that will prove way better for both Earth and us, won't it?
Of course, I'm oversimplifying serious matter for which I have no answer, but that's just to say that at some point one has to stop and look at the actual individuals, and wonder "should I think only in abstracts, ignoring the real, tangible human element?". Or to put it in a more unfair way, can you really ignore the suffering of that little girl who's a slave for no fault of her own?
I'll point out that possibly I might, and indeed in some matters I do, even in the real world.
I'm only criticizing you because you raised your point as you did - if someone had put it the other way around, defending the actions of the MC, I would probably be arguing the opposite.
Ah, and as a final note, I might also add that I'm not entirely sure it really is the best choice.
Yes, generally speaking to not engage in a trade you detest is a good idea and negative for the trade, but depending on how big and entrenched in the society that phenomenon is, it might be useless, and in the long run you could just get used to ignoring the issue, seeing as how most efforts come to naught, and so grow apathetic to the whole issue. Yes, you still won't be contributing to the trade itself, but you also won't be doing anything to stop it. On the other hand one may imagine that each slave that is bought and freed, especially if the person doing the act is someone who actively opposes the practice, will be one more person capable of fighting against it - and the person freeing the slaves as well will live in an environment more conducive to his wish of eradicating slavery, and indifference will hardly become an issue as he surrounds himself with people who fight alongside him.
Of course that's not final, and as I said before, and as it is for every issue, there are both negative and positives in both approaches.
In the end this is a wish-fulfillment isekai manga, with a guy who became a slave merchant because he didn't want to take an active role in anything and just live a lazy life, so what's the use in taking it seriously and discussing about the matter in stupidly long detail, uh?
...
Wait, why did I write this wall of text again?