First: you need to spend credits in the Shop to open up the Private sell and Public Sell option.
Second: Go into your collection (Make sure the card or cards you want to trade are not in your deck) And click on sell this character.
Third: Choose Private Sell, enter the persons name you wish to trade the card to enter 50 Clintz and then enter your password tht you login with.
Fourth: The other person will do the same thing, the cards are sold to each othere for 50 Clintz each, which in the end you will both get the cards and your Clintz back in return.
This is an At Your Own Risk Thing to do. Cause The othere person may not be able to do Private Sells or Public sells, which will leave you with the 50 Clintz per card they get and you loos out on the deal. I Lost 2 cards today due to this. I lost a lvl 1 Ambre and a lvl 1 Aurelia. I was supposed to get Vermin N and Flyer but didn't.