TBH, treating slaves as a commodity and simply a trade "good" would likely rub people the wrong way, and garner a lot of negative publicity and lost chances for sales. That really just glazes over the whole issue entirely, and dehumanizes the slaves into the same level as a crate of bananas or barrel of monkey oil. Even if you model deaths and suicides in, it's going to come off the same way as if several crates of food spoiled or was tossed overboard during a storm.
When thinking of writing in a delicate subject, first ask "Am I being respectful of what these people went through?" Then ask, "does this add to the story and/or gameplay?" If the answer is not truthfully 'yes' to both, don't put it in game.
I have a scripted instance that could result in a rape (depending on whether the player acts to intervene or not), and another quest line that features slavery. But, in the first case, it's more providing the player with a definite choice with consequences, as you will see later the results of your inaction. The latter I'm not happy about, and may cut it even though it would fit with the setting and overall story. In short, the player is investigating missing persons and stumbles across a slavery ring operating in a refugee camp. In this case, you see the results of not investigating and dealing with the issue more at a distance, as you're not really introduced in any fashion to the victims unless you choose to follow that quest line. I'm not happy about that, as I'm not sure how to push forward the sense of what happens if you don't care about these people. Even if they're not real, I want the player to feel a sense of unease just leaving them to their fate.
In both cases, if I do not feel that the characters in both arcs are humanized enough, I'll cut them entirely. The former is in a much better position, as you meet the NPC and others around them repeatedly, but the latter so far is just to distant from the actual people involved.
In your case, it looks more like 'slave' is just another line item in a list, which comes off unintentionally as dehumanizing. And given your gameplay, I'm not sure how you could humanize the slaves given that you're operating at a more abstract level anyway, and not really driving a heavy narrative.