If you go to your collection screen you should see a button which says sell this card under every card. With you click on it it will give you options to sell it on the market or to a certain player or kate.
You could also start a secure trade by clicking on the secure trade tab in the collection tab.
p.s you need to buy credits befor you can trade cards. Aaa-battery1 15:22, September 26, 2011 (UTC)