In general, the slaving houses would not sell a slave to a person who's liable to cause them trouble.
For example, A very Rich independent merchant who has a lot of coins, but no bodyguards, no personal guard, no estate, no compound, etc. He could actually afford a slave in theory. But he cant afford to keep control over that slave. Kind of like buying an alligator. Zoo and other organizations could probably get it with ease. A normal citizens will likely have some severe difficulties (Or should anyway). Because if you dont feed that slave, you will drive that slave to brigandry. Brigandry will lead to escape, and escape will lead to a bad example for other slaves.
So it's rare that slaves are sold to normal citizens, or even individuals really. More often they're sold to organizations with long standing reputation of being someone whose slave ownership is not causing too much headache to Slaving Houses and Templarate. If AoD keeps needing to ride out to corral your escaped slaves, maybe its best not to sell them to you.
The whole Shtick of Borsail is that they breed slaves. Which means those slaves do not want to escape and can be trusted to do tasks of complexity and confidence. It's rare or neigh-impossible for captured slaves to be in this position.
Understand, that Allanak is a society hugely based on slaves. Most of it's industry is done by slaves. If the slaves were to revolt, the Templarate would have to use massive destruction magick to put the revolt down, likely losing a major amount of slaves. The biggest thing that keeps the slaves in check is "the way things are". The societal order, the way people intrinsically understand that Nobility are "BETTER" then common people and slaves are property. Even Slaves themselves believe this to be the case. And that kind of psychological make up is what keeps Armageddon slavery viable. This means that a commoner being a vagabound that spouts out stupid shit is bad, but ultimately inconsequential. Eventually he does it in front of someone else and gets his head separated. Dissension within slaves is a lot less tolerated. A lot more effort is put into preventing this. To maintain the Status Quo = The Cheapest method of keeping people obedient.
There was once a time when enslaving someone was more common. Templars would enslave PCs, nobility. Heck, as a Desert Elf Raider Tribe, me and my kin captured three PCs and sold them as slaves to House Tor. That was awhile ago and provided for some really fun dynamic ... short term. Long term though, slavery is difficult. For people to be genuinely enslaved, their freedoms must be limited. Otherwise they're just ultra-loyal fellow citizens. And if their freedoms are limited, then their ability to go out and persue fun things to play around in a mud is limited. Which means their owner has a RESPONSIBILITY to entertain them. Which is a lot extra stress on the owner. Usually it all resulted in a lot of bored/anxious players and storages.
There is actually a rule somewhere that you cant even perma-imprison anyone for a long while, without ... discussing it OOCly with your captive? Because if you're holding them in a cell, you're responsible for their entertainment in the game. Am I making this shit up?