I've been trying for quite some time to puzzle this game out with little to no success.
-I tried out the normal starts first, simple difficulty. More often than not the time and resources it took to train a single slave outweighed the sales price by quite a bit. So even when I DID manage to sell them (after butting my head against them for 4+ decades) I would still be operating at a loss. (I haven't messed with loans yet - in fact I haven't even found where you're supposed to take one out.)
-Normal starts frustrated me, so I went for the custom ones instead. Seems getting them to obey is even harder there. Eventually I managed. But ran into the same problem - selling slaves did not even let me break even.
-Making slaves presentable is another challenge. Some combinations are simple enough - ballerina outfit for artists, sorceress robe for witch doctors, etc. - but some don't make sense. What the hell kinda outfit am I supposed to put together for an assistant?
-Went for custom easy starts. STILL haven't puzzled out how to lower the chances of rebellion, or how to quickly decrease thorns of rebellion. And on top of that for some reason their Pride started going up seemingly on its own while Temperament and Nature go down?
Since the Wiki's outdated I'd love to get some more in-depth info on this game that's up to snuff.
There's definitely a lot to the game but once you get used to how things work it becomes very manageable. It's just a learning curve to get there.
So normal (and especially easy) makes things simple by giving you a large sum of upfront cash to play with which means you can take your time learning. Even if your first few slaves are sold at a loss, it's not the end of the world. What's important is you start to feel more comfortable with the training process first and foremost.
To start with, unless you're using drugs, alchemy, magic, or a slaver with very high starting skills, then nearly any girl you start off with is going to have a fair bit of rebelliousness to her in the beginning. That begins to improve as she develops habit, taming, and awareness which basically means you need to have a little bit of patience in the beginning. You set your rules, try to find things she's willing to do, and punish her for the things she's not. Trying to do too much too soon such as multiple repeated or particularly brutal punishments can create issues with depression and risk mental breaks, suicide attempts, or attempts to kill you in your sleep so again, it's better to do things gradually... At least while you're still learning. Don't expect her to become obedient or willing to do sexual things with you in the first few days.
When it comes to making them presentable, honestly I think you're overthinking it. Buying them certain clothing can help with their mood, or with training specific skills, but for most low level slaves you won't need to do much more than give them a full treatment at the spa which will raise their charm high enough for you to satisfy the requirements for most sales. If you're trying to sell exceptionally high quality slaves in the A or S ranks then finding ways to increase your style and charm become more important.
Which begs the question, what quality slaves are you attempting to train for at this point? And how are you planning to sell them? To a specific buyer? At auction? Guild contracts or subpurchaser? It'll be easier to give you more specific advice when I know what you're trying to accomplish.
What rules have you been setting for them? What techniques have you been trying that haven't been working out for you?