one thing is to live close to the end of a society with slavery or on the beginning of a society with slavery, the thought that its wrong or the desire to end it is almost common thought.
But to live on the middle of the time period is a different thing, it's part of their culture, it's part of the society, it is acceptable to everyone, fighting against it is insanity, it's the same to say that earth is flat, you will not convince anyone, because everyone knows earth is not flat, is it wrong to buy slaves? Yes it is, you are feeding the system and making it worse, Rudeus could find other solution? Yes! But he wanted to blend with society, his superior thinks its ok, his friends thinks its ok, society thinks its ok. So it's the easier and more natural choice to be made.
Now some food for thought:
Why slavery ended on our society? Was because we notice that it was wrong and out of nowhere it was ended? Was it ended because of rebellions and we started giving more value to human life? Or was it because the slavery system accumulate too much wealth on a very small group of people that had no way to spend all that money, and as a consequence the value of money decreased super fast, society had less people doing business and less money in circulation, and as a countermeasure to fix the value of money and to prevent the collapse of society it resulted on the end of slavery ?