There needs to be a way to sell multiple cards in one go! Like instead of the sell button below every card, there is a check mark, and at the bottom of the screen implemented is a "sell all" button :D
Energy refreshes quite fast ;) In the meantime you can improve your deck and do some Duels against your friends in your list and some duels in the Duel Arena :)
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that would possibly lead to much more unwanted sales ;)