Actually, this game is not a TCG, it's a CCG (Collectable Card Game).
Also, trading cards will break the game in more way than one and thus I personally object against such suggestion as it will not improve the game's health. In fact, I'd say it will do the exact opposite.
Unless the trading system is worked out in such a way that there are no exploits (like trading a Pixie for Battle-wise Athena should simply never ever happen).
If you're talking about how players may exploit the feature, in order to get multiple event pirates, (aka, if I used two Apple iOS devices to use two accounts to gain 2 SR Event pirate, move one to the other, etc...) just make some of the event pirates "Unique", or "untradeable".