I assumed by looking at the cards it would be like a game of 'Top Trumps'. If this is right even if Maxis do sell the cards i would assume it won't be very expensive.
Remember though that there is going to be literally millions of creatures made, not to mention planets/plants etc. Do you really think that Maxis could make a printed card for every single one of them? I very much doubt that there will be printed versions of the cards, and even if there was i would expect it to be limited to Maxis designs and maybe competition winners.
More likely it will be a mini game that can be played on mobile phones, handheld consoles etc. You would download the content to the device and then find a friend who had also downloaded there choices to their device. Battle would commence (maybe in a top trump stylee) and the winner would get to keep the cards that he won. With a bit of luck this will extend to an online pc mini game that others the same thing.
Then again fell free to ignore anything I wrote since it is all guess work and conjecture.
