If you are on PC, you can cheat it a bit (if you need Iron ingots now! for your armors). What ya do is select the vendor, and mouse over an item. Then take the mouse and move it towards his name on the left, and then move it down to your name. It should now show the item in question, while you are 'over the line' and into your inventory. Then just push [enter] a bunch of times, and it's like the item is yours, and you are selling it to the vendor. It's a good way to get the materials out of the vendor when in dire need, if you aren't into spending hours gathering materials yourself.