I've been thinking about this for awhile and slaving needs a change, badly, I advise we keep the ability to sell slaves to megtzer/vortis for 100-200 caps but there need to be certain changes.
Slaves should be split into different categories, Metal Armored, Leather Armored, Leather Jacket, Ghoul and the POS slaves that are only used for mining (that includes child labor
)
Players should be able to put an order in at Megtzer/Vortis how many of what type for how much and then will have the money deducted from their person
Only slavers can see what orders have been placed and by whom (in case it's someone you hate and don't want to help out and that can also help gm's track anybody who is just spamming slave orders and ruining the system)
Slavers can then decide to do said slaving for the people, return to metgzer/vortis and tell them which order they should be put into and receive the caps for the slave.
If the buyer changes his mind or no one has bothered slaving for them they should be able to withdraw the amount of caps they were offering and receive their money back, unless of course a slave or two was bought in which case that amount will be deducted from what they get back.
As it is, slaving is mostly just selling to Metgzer/Vortis with very little human interaction unless you come upon the random person who needs to buy a slave for mining.
If that idea is terrible how about giving slavers a pen (imaginary) at Metgzer or Vortis's where they can keep 5-10 slaves instead of having to sell them to an NPC to be able to slave more (there could be a fee for using the pen or make it a one time buy for however many caps seems fair.)
Also, how many of what type of slave could reflect Metgzers stock where the price goes down the more of X type of slave is sold to him (Metal Armor slaves sold, Metal Armor slave price goes down)
Comment and I will tweak the suggestion as needed.