Why isn't my trading GUI working?

Ok, so I'm making a mod with a small wandering-trader like mob. It has it's own GUI and item that it gives.
(The trade is 1 Apple -> 1 Spring)

The trade button, upon being clicked, should take away one apple from the player's inventory if they have one, and if they do have one, it should give a spring back.
It seems simple. But for whatever reason, the procedure just isn't working. It won't take away the apple or give the spring, even when I have 64 apples in my inventory.

(I don't see anything really.. wrong with the procedure. It's activated when the button is clicked.)