I created this addon to help me decide which items to bank/vendor/trade.

Item tooltips are modified to display icons of the professions that can use the item as a crafting ingredient. The trade items from vendors aren't affected (spices, vials, threads..) 

Additonal features

Display a quest icon within the tooltip to indicate if the item is required for a quest (only the tradable items such as beast parts, frost oil, hillman's cloak). Classic has numerous quests that require trade items to complete (these quest items might have some demand/value on the AH)

Configuration dialog which allows the user to control which icons are displayed and modify the icon size (see images)

Track items that can be exchanged for Darkmoon Faire Prize Tickets (these items may have some demand/value once the faire becomes available)

Retail port

The port provides coverage of each expansion's profession items (9.0.2)