Like Boradam said, you put what you want from his inventory on "the table", then you put what you want to barter from your inventory on "the table", then you see below them what they both are worth and they will have to match, so you add and subtract items till you get to a good value and your barter. That's the idea of bartering. It's not that you chose the items you want, and then pay the exact amount in money like we do nowadays... although you can do that aswell, but you can't come into a clothing store nowadays and try to trade your father's old watch, a silver necklace, and a box of all Police Academy films for apparel
Although that would be cool