When you are in your collection, click on any character you want to sell
privatly, go to "buy/sell" icon, sell, then set a price and select "private",
you will be able to put your friend nickname to put the card on sale
just for him.
Many players set up trades this way.
Note that to prevent abuse of the system, you'll be able to sell on the
market or privatly only after you made your first purchase at the shop
(then it's unlocked forever).
You can learn more about this in the "Game Help" page (link is at the
bottom of the left menu).
Don't forget to sponsor your friends to the game as you will get free
credits everytime they buy some! (see "invite your friends"