You're stuck on the idea that slavery in any form and at all levels is pure evil, and are refusing to use your brain at all in this conversation.
I am using my brain, not just emotions. Basically, I'll repeat that participating in the slave trade feeds it. It doesn't hinder or reduce it.
For one, your comparison with other situations like a house fire is ignoring the complete difference with the situation we're discussing.
(As a sidenote, yes, I'd prefer you walk away from a fire if you don't know what you're doing. Otherwise we'll just end up with two victims instead of one. There are many situations where not intervening directly is the right choice. Calling emergency is often a way better move. Also, this wouldn't be "invasion" if I called for help. You're making up a completely stupid hypothetical.)
Here, we're not talking about saving a person in danger from an isolated incident that is the result of either an accident or a crime. We're talking about an institution that is morally corrupt, but legal in that context. (Morality and laws are - sadly - not directly correlated, even in the real world.) "Saving" one individual, which already is not really saving them for reasons I discussed previously,
will condemn more as the demand, as in "offer and demand" which is the context I'm using the word, increases the incentive for more offer. You accuse me of not using my brain, but you're the one ignoring the larger impact of your proposed actions in favor of an emotional and isolated act of self-righteousness. As long as the institution is legal, this is contributing to it, not ending it, no matter how many you "save".
Basically, there is a reason that people in the real world call to boycott unethical products rather than attempt to buy them all.
And we're not talking about walking into a bar and ordering a cocktail as a way to initiate conversation with the barman. You're mentioning how "just walking in and asking questions" doesn't help, but all he's doing is asking for slaves that meet a certain condition ("war slaves, elf and beast person"). Normal "customer" behavior so far. Why would he need to buy any slave to establish a better relation? More importantly, does he need to do this with every trader? He will run out of money very fast in this case. Which leads us back to self-satisfaction as the only motive to buy slaves. And the way you present it, he wouldn't even save the weaker slaves which are more at risk but the stronger ones who would prove useful to him... but would likely survive longer as a slave under a different owner anyway. So your proposal is pure participation in slavery, not "saving slaves".
Also, the MC didn't intend to walk to slave traders just for the fun of acting indignant. He has a purpose which is way more relatable than a grandiose "attempt to abolish slavery". He wants to save the friends of his friends. He doesn't like it, but he wants to help people he knows. It's personal. It's relatable. It's understandable. Not every MC needs to save the whole world at once. And not every MC is an OP juggernaut that will get away with upending society or run on unlimited funds to save anyone he fancies.
To conclude, I already had a message before on how slavery is always evil to some degree. A lesser degree when you treat your slaves well, but as long as you don't actually free them, slaves are just a step away from being treated inhumanely. In this case, the MC is going to keep traveling so he's going to fight monsters on a regular basis. Any slave he buys will be involved in fights to the death, will it or not. Talking to them nicely doesn't change the fact that he's making them fight... for
his life.
I can argue a lot more about this, but I keep to my opinion that he acted the right way. So far at least.