First, thank you for your feedback !
Cards attack a random enemy (I just added it to the How to play info).
Some cards/heroes have "On play" effects. If you were able to put cards back into your hand, it would be too powerful. (It's a gameplay choice here.)
Bots play like you: rolling to get cards, buying them, selling them, and trying to build a better board following some rules. BUT, players in a game don't share the same card pool for now. A card just has a % chance to show up when you roll. I'll probably add a shared pool in a later update.