Sell Value
 

Hopefully I haven't just missed it or don't know how to use the normal UI properly but.....

A mouseover function for any item in your inventory.
Once you have sold an item it logs price in a database (averages it out dependant on where you sell item and for what price)
The mouseover then tells you the average vendor price for that item.
For me, when bags are filling up, it tells me what are the best things to dump etc. (I generally hoard everything til I run out of room)

(Hate to use another game but WoW had a mod called 'sell value' that did this quite well)

not possible, even though it keeps getting requested every other day.