At first I found the bartering system to be a bit clumsy. Either click and drag to the selling bar, or double click an item for the same effect. I hate both of these options. I discovered recently that if you have your inventory open, you can shift click an item to mark it as "wares". When you do this, you should see a silver coin icon appear above the item in question. Then, in the barter screen, click the "sell wares" button at the bottom of your inventory and presto, all of the items you tagged are added to the sell list. Hope this helps.