An auction system would be a great idea, once more cards are available (20+ per element would be ideal). Adding a minimum amount for any card would prevent exploitation of the system between friends, and adding a percentage tax to the final price would keep the economy in check. Also, the selling price of upgraded cards should be dropped from ~1100 to ~750 in the Bazaar. Both the tax and the lower selling cost would keep the Elements economy in check, and prevent people from eventually amassing hundreds of thousands, or even millions, or Electrum.
(This is an observation based on what I've seen in many MMO's, predominately MapleStory, Perfect World, and Dream of Mirror. Limiting the money making capability and taxing the trade system helps prevent the economic structure of the community from growing out of control. Of course, this only happens when there is a free exchange of money, such as a trade system. Since there is currently no way to actually exchange cards and money, the reduction in card selling prices shouldn't take place until a trade system is up and running.)