"Diamonds - A number card of diamonds can be used to increase the player's defense for a single enemy's turn by the number on the card."
I intended for it to mean that, when your enemy is trying to attack you, you play it to raise your defense (AC for D&D).
Also, I'd personally not allow my players to trade cards. Four players, four suits, five cards each, some scheming, and now everyone has a flush and some might have better. If I did allow trading, I'd put a limit, something like one trade per level, per player. So two players could only trade once with each other upon leveling up, and with no one else until the next level up.
And DM heir? I don't quite understand the question.