There is a lot of interesting discussion and analysis to be had here, so let's not stop short of having it.
The initial question is does a depiction of a thing, such as slavery, automatically condone the thing? Does "Apocalypse Now" advocate for war? Does "Lolita" encourage pedophilia? Most people agree that the framing is more important than the content - to make an audience feel the visceral cruelty inherent to these practices or (in Lolita) to wrestle with the cognative dissonance they create is to hold the horror up to the light for critical examination. War, slavery, pedophilia, abuse, these are all challenging subjects and to excise them from your consumption of media is to deny that they are worthy of consideration, in a way condoning them by default by accepting or disregarding their existence. I find it interesting that Zephyr247 draws the line at depictions of slavery when war, patriarchy, and abuse have all already been depicted or referenced in this series and are all contemptible. So lets dig into the framing and the fantasies of the genre - and this work in particular - to try and suss out why the narrative has taken us to the slave auction and why this subject might feel different.
The narrative has established repeatedly that our protagonist is in a dangerous situation surrounded by threatening and abusive men. They exercise their power over her, push her around, verbally abuse her and encourage others to abuse her. Any proportional reaction of shouting or screaming on her part redoubles their abuse; the protagonist is forced to become meek and servile to survive. Her anger and resistance is contrasted by the 'heroine' who already fits into the patriarchy nicely by naturally conforming to this role. If this is a fantasy intended for the titillation of women, why is this the case? Well there are a number of reasons. It is unfortunate to note that the reality for a lot of women is one of abuse at the hands of the patriarchy, so this element makes our protagonist relatable to female readers and helps settle us in a world which we are familiar with. A lot of women are also attracted to an element of danger in their partners (which in theory advocates their strength and fitness, which is why serial killers have fangirls), and media such as manga is a safe way of exploring those feelings without suffering the actual consequences of being with an abusive, dominating man. As I mentioned, however, the narrative has established that for our protagonist the stakes are 'real' and most people agree that enduring abuse is not acceptable in reality. The safest romantic partner for the protagonist in her situation is a man who she has an equal or greater power dynamic with. Additionally it allows the author to appeal to a broader audience by integrating an empowerment fantasy in an otherwise disempowering one. Given the disempowering nature of her situation the only leverage the protagonist can bring to bear against someone is her class. Enter the slave market.
I'm not going to argue for the ethics of romance in such an imbalanced power dynamic as the master-slave or master-servant, but it is fair to say that it serves to place our protagonist in a position of power over her partner thus narratively giving her a better chance of survival and meta-narratively appealing to the female empowerment fantasy. Having established its purpose in the narrative we can examine its framing in the chapter to try and decide whether the author uses it responsibly. As Zephyr mentioned our perspective - and by default our sympathies - rest with our bouguie protagonist who sits in the safety of the audience watching Iklies become the victim of systemic and physical violence. Based on her facial expression and shock it is obvious that this is not something she approves of, and her sympathies - and by extension our own - are with Iklies. Additionally our perspective in the final panel shifts away from the protagonist to Iklies, his face of determination and resignation one we might recognize as a mirror of expressions our protagonist has made. Both have been victims of violence and both are going to have to adopt meek and servile mannerisms to navigate their reality. The author is drawing parallels between the two, which helps to establish our investment in their potential relationship.
So far none of this is an endorsement of slavery, but it could be considered an equivalence between literal slavery and the abuse and lack of freedom of slaves and the figurative slavery of women and their abuse and lack of freedom in a patriarchal system. Whether you find that acceptable is a matter of personal opinion and so not in the scope of this discussion. So the question then becomes in order to achieve the desired outcome, what power does the protagonist have to change the system and what agency do they have to exercise within the system? As @\Amairani brings up, it is unacceptable to financially support the immoral institution by outright buying him, but to go against the institution which buttresses the nobility would pose a very real threat to her family, reputation, and life. She has no power to change the system, and as she has no skills in thievery nor ability to leverage her position while 'anonymous' she likely has very little ability to engage with it other than in the manner prescribed by its rules.
So now let's consider how the narrative might have differed. The author could have had an opportunity to introduce Iklies as a servant at the juncture where her maid was in question. Removing her would have opened the position which could have been a promotion for another maid within the house and required an introduction of another servant, such as Iklies, to take their place in the lower ranks of the servants. This would have allowed him to be a nominally free albeit low-class man which is a significantly higher position relative to slave. It seems a neat and tidy solution but whether it would have been a better narrative decision will depend on what the author's intentions for the relationship between the protagonist and her handmaid are, and it seems that dynamic is still developing so we might have to reserve judgement. Alternatively because his slavery and purchase by the duke were stated early in the work Iklies could have purchased by him as a guard for the protagonist after her assault by the prince. This simply would have shifted the slavery off screen in a manner to the way the war, and the destruction and subjugation of Iklies homeland, has been off-screen. I think it is worth asking ourselves if we find that solution preferable - pushing it off screen without further comment - simply because we don't have to give it further thought; don't have to face its horrors to condemn it.
I hope you enjoyed my novel. It was fun to think about.