There are 3 ways to sell slaves which benefit from the Slaver specialization.
- When traveling outside, the party menu can be accessed with the "Control" button (I don't know why it has that name), and in the bottom left is the button to "Quicksell Captives" for half their current value.
- You can sell any slave to any slave guild; there are 4 slave guilds, which are located in most of the large towns. To access the one in the same town as the mansion (should be Wimborn, unless you have Leo's mod), leave the mansion, press "Visit Slaver's Guild", press "Offer your servants", and select the slaves one at a time to sell.
- You can give slaves to fulfill requests, when in the slave guild press "See custom requests", select a request to accept and then select a slave to give. Note, you can only accept one quest per slave guild, and there is only a 33% bonus from Slaver spec compared to the 100% bonus normally.
Edit: added 3rd way