because unlike a standard material commodity, a playing card's value can be dramatically affected by whatever the publishing company decides to do to the game. whether it be printing more of the card, releasing a new version of the card that's more powerful, making that one obsolete, or even discontinuing the game altogether.

i'm sure you COULD make something like this, but really with the value of the item being intangible and largely based on emotional/prospective value, it'd be hard to get traction with the idea...

<disclosure: i know nothing about WoW cards>