There used to be an addon be it third party, ashita or windower that would pull data from FFXIAH whenever you hovered over an item in your inventory and it would display the sell history for that item essentially telling you how much every item is worth without looking it up.
Windower has one on its site but it's broken and the creator is MIA: http://forums.windower.net/index.php...r-replacement/ It doesn't recognize FFXI instances.