You have to work on the main quest.

The last part of the main quest before the mind reading system becomes fully open to you is to do a mind reading on the guy that the ransom brokers sell to. After you do, he'll tell the ransom brokers to buy prisoners from you at full price.

You do get a prize for selling enough prisoners to him, though he always buys for 50 like Ramun. A hammer that's too slow to fight with, but it does give 5 strength when equipped which can be helpful for getting more Ironflesh/Power Strike/Draw/Throw.

Thanks ^^