You need to have dialogue that calls: akSpeaker.ShowBarterMenu()
That will show the trade menu.
As for making them sell stuff, they will probably automatically sell whatever is in their inventory. Most merchants also have a faction that is linked to a chest that contains the shop's goods. However, I've had problems getting that to work. As far as I can tell, you should just have to create a faction, add the chest to the Vender tab and set up some of the things in that tab (what type of vender). Then add the merchant to the faction. But it hasn't been working for me.