Hadn't thought about that. Perhaps we should only allow trades within a certain type? I.E. Pillars can only be traded for pillars, basic cards for other basic cards, rares/upgraded cards for other rares/upgraded. Could still be abused somewhat by trading, say, a Dragonfly for a Phase Dragon, but helps a little.
Optimally, we could track the money received by selling starter cards/rares and not allow cards purchased with THAT money to be traded, but that seems a little excessive and probably a headache to implement, not to mention explain to people.