You give them items that contain Lucrum, such as Silver Ingots/blocks and Gold Ingots/blocks/apples/armor. After you give them enough Lucrum, they'll trade with you. The more lucrum you give, the better stuff(on average), that you get but anything over 8 lucrum is a waste.The pech (think that's how it's spelled). Thaumcraft critter. If you give them... something, they'll trade with you or something like that.