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PerfectDeath
That's not how slavery works, this manga has already stated that in world there are three types of slaves: sex, labor and domestic. Sex slaves do not go up in value after use, they may get better at sex but age is an important part of the market value and it goes down with time, and if you're going to buy a girl, why would you buy literal used goods? Training her yourself would be part of the fun. If you wanted experience prostitutes exist, go to one of those. Labor slaves are not for complex tasks, they do repetitive labor inductive tasks like moving resources, simple farm work, things like that. You don't teach the laborers skills, you teach craftsman skills, but slaves wouldn't be involved in that business at all because those tasks are more complex and it's more valuable to hire and train someone then it is to give it to a piece of property, usually a son in this setting though guilds are also a thing. Last is domestic, all tasks done are simple repetitive not labor inductive tasks, anybody can do these tasks, they just get slaves to do them because they're boring and take no skill.