http://www.havenandhearth.com/forum/viewtopic.php?f=35&t=31356
Check link under point 3. To shorten it up, everyone uses value for certain goods, so it easier to value up any trades that ocur. That is why most traders have similar point value of forgables, pearls, etc + have put their own value on their goods for how much they are selling it.
Hope it helps.