I don't know about any criteria outside being able to outbid everyone else at the auction. Who honestly cares what you do with it as long as Borsail, Kasix, or any of the other virtual slavers gets their money.
buy some cheap injured ones off a shady raider group at an abandoned outpost, tie them up to walk behind your mount, use as pleasure slaves and then as bait when you're done with them.
Save up enough to buy one from borsail just to show off your wealth and status as a 'sir' or 'miss'-worthy commoner and then find out you can't actually afford the maintenance, like your poor neighbor and his mercedes. NOBODY cares if it starves because of you.
Buy one to be your spouse and live in your apartment to keep it clean, cook your food, and pleasure you. When you can't afford your rent, it just follows you around carrying your stuff. When you can't afford to feed it, sell it.
If your crazy murderer doesn't want to go to jail or piss off the wrong powerful group, buy a person to torture and murder in the wilderness. STAGE YOUR OWN MAN HUNTS. It's an expensive hobby, but those are the best hobbies.
If your minor merchant house takes off and you can afford a warehouse, buy slaves to maintain it and to do the menial low-skill virtual labor. Gather them up all at once to go out and forage for rocks as a team under the supervision of your whip.
These are the ways slavery works. It's just ALL virtual right now.