Thanks for the feedback! Yeah the main issue there is when I was working on the game I had only one card of every rarity as test cards, so a total of 5 cards. So as I tested it, it felt balanced to me. Then later on I realized that adding more cards would upset the balance, since you have less and less chance to have the cards the kids want, so I just raised the prices of the cards in total, so you get a lot more if you happen to sell a card.
But yeah in the long run I plan on changing how the kids ask for cards, and also adding more systems for selling cards in case no kid wants the ones you have.