PC-gamer interview with Jakob Porser <--- Seems to give very good information on the game.
It says that some scrolls you'll only get by playing Single player, and some scrolls can only be bought, but they can all be sold and auctioned with the ingame currency.

Seems like a nice way around it if you ask me.

Edit: And if you've played other card games, some cards are good if you have them in a combination with others, but they might suck alone or in the wrong combination, so. I like the idea of the game.