Thank you for trying our game!
We'll have to figure out a better way to communicate this. Basically you have to sell a card to buy more cards. But if you sell more cards than you buy, the game will refill your deck with low-tier cards. This is to ensure that your deck is always at 20 cards.
There is also a bug mentioned in a previous comment by my development partner that causes your deck to exceed 20 cards that could have been preventing you from buying more cards.
We plan to fix and add content after the jam voting period is over.
Thanks!