> Firstly you need to have the skill Prisoner Management to 1 at the minimum.
>After each battle you will get the chance to imprison any enemies that you have knocked out, rather than killed.
>Simply ensure that you have captured 5 enemies in your party and look in the taverns for the Slave Trader OR a Ransom Broker.
>Speak to the Slave Trader/Ransom Broker and tell him that you have men you want to sell. Select your prisoners and sell them. Sell 5 and the achievement will pop.
Important -
>You cannot sell named characters like a Jarl or Count. They must be regular units.
>The slave trader might take some finding, make sure that you go upstairs in any taverns to look for him.