Situation: Player A has 4 trinkets and a fruit, player B has 1 fruit and 1 silk.
Player A wants to play Shaman to trade trinkets for fruit. Is this a legal play? There are only 6 fruit in the box.
Why not? You are trading in 4 Trinkets to received 4 of anything besides Trinkets. You have 1 Fruit, your opponent has 1 Fruit, so 4 remain in the supply and it is a legal trade.
"Different" just means something other than what you are trading. Not different from what you already have.