When trading with a villager, if you don't have the item in your inventory, it doesn't create a new item in your inventory but still takes your items. For example if I give potatoes to a farmer for emeralds and if I don't have any emeralds in my inventory, then giving potatoes doesn't give me emeralds even though my potato count declines. Also, when you max out a slot say I have 64 emeralds, then it won't take over an open inventory slot when I get more.