There isn’t a direct way to know the rarity of every tradable item. At least not from the trade window. I think this is necessary. This information can help players to focus on selling rare items. Selling rare items is more profitable for everyone (players buying, players selling and devs). Something like: common, rare and very rare should be enough. The rarity of each item can be taken from the actual percentage with which the items appear in loot.
The only drawback i can come across is some players will realize than some items are common and hard to sell by 5000GS.