Help with trades (nbt tags specifically)

So after making a mob and a special gui for trades, I finally managed to make that gui open when the mob is right-clicked.

However there is a problem. 

When I place the required "currency" for the trade, the item I'm trying to buy won't appear (in few words the procedures won't work).

I've watched tutorials on youtube that require custom nbt tags for the trades, yet a message appears that they might not be properly synced.

I'm completely oblivious as to why the nbt tags won't work, if someone is willing to help me make them work, I would be very grateful

Thanks in advance