The only cards that would make any sense to trade are rares, because getting electrum isn't as hard as it sounds after a while, so you can get everything else from the Bazaar. For rares other than nymphs and marks, getting a friend to use a farmer deck in pvp is more efficient than trading the cards.
Because of the oracle, trading nymphs is abusable. Marks can only be obtained from codes, good luck findig someone that wants to trade that. That's why adding a trading system is either abusable or completely unnecessary.