Zendar is neutral, so you can visit it no mater wich faction you sign with, so is the salt mine. In Zendar, as you enter the town, go forward (away from the trainer) until you're near some stairs. There will be some guy, a slave trader, talk to him and you will be able to sell your prisoners. In the Salt Mine, as you enter, go to the man to your right and talk to him, he'll buy your prisoners. The price for a prisoner is 100 everywhere, except if you're on a quest to deliver let's say 6 army members from the opposite faction.