Since the same group that's translating this manga has already translated the light novels, I went ahead and grabbed the first one to see if it had any additional insights.
First, the novel really ain't that good. The manga's excellent art is doing a lot of heavy lifting for this story.
Second, the slave trade is pretty much just speed-ran through. Tohru finds it gross but after what just happened with his original party, he wants a companion who cannot betray him or disobey him. The slave trader initial offers priced in the millions are a lizardman to use as a meatshield, and then an elf mage as both a living weapon and as a top tier fucktoy.
Tohru revealing that he's only got 150K gets him shown to the animals and sick/defective/unruly children section, since they're worth far less, but still run for 100K or so as future investment. Kaede turns out extra discounted because she's terminally ill, and has no value even as an investment. And we know where it goes from there.
Third, to my surprise Tohru actually asked Kaede how she ever became a slave in the first place (I'm so used to MCs showing absolutely no curiosity or interest in their waifu's backstory) but they get interrupted by the bandit attack where they rescue that ojou-sama girl and of course it doesn't come up again. Maybe that still happened in the manga and I just forgot about it?
Also, here's a goofy thing. The manga definitely tries to play up how Kaede's mature beyond her years, but the novel plays up that that Kaede's a child with an adult's body. (Though I guess being 15 and magically aging-up to 20ish is less weird than Raphtalia being like 10 and aging up to 18 or whatever the heck she's supposed to be.)
I'm also amused that a spoiler that I'd seen about the demon king is actually set up by a casual aside in the prologue of the first novel.