Agree - undo button is very helpful to new players. They may click 'confirm' without knowing the consequences of their decision until after realizing they bought the wrong thing. Undo is beautifully implemented in Pax Pamir and Ark Nova, and if in the cases where undo is unavailable (like getting a card from the deck), it tells you this cannot be undo'd.
Not having an undo is more painful to new players, and I believe Paverson Games intention with this BGA implementation was to get new players into the game. My two cents.