Decrease food
Events
Unit - A unit Sells an item
Conditions
Actions
For each (Integer A) from 1 to MaxArray, do (Actions)
Loop - Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Item-type of (Sold Item)) Equal to (==) Item_Type[(Integer A)]
Then - Actions
Player - Set (Owner of (Buying unit)) Food used to (((Owner of (Buying unit)) Food used) - Food_Cost[(Integer A)])
Else - Actions
Just make sure you set the gold and lumber cost for the items to 0 and put in the description how much food the items cost.
You also might want to use a variable for the for loop if you're already using Integer A in one of your triggers.
(Haven't tested it yet but I suppose this works)