When trading for objects other than emeralds, after pressing the desired trade on the left column of the GUI and completing a successful trade the object does not go down into the player inventory emeralds are taken from it. 

If the player can drop emeralds into the trade input slot, then click on the output slot, the item can then be moved to inventory. This is  to always possible to select because the mouseover selection defaults back down in most cases before the player can take the traded item from the output slot. 

Also, sometime the game freezes and will not unfreeze when the trade GUI is open.