IF a trading system was ever introduced i would think limiting the amount of trades would be an option, no one will ever trade common cards, the bazaar is way better, but for rares it can be done like this
only trade rares (weapons, pharao, squid, miracle) for other rares
only trade shards for other shards
only trade nymphs for other nymphs
only trade marks for marks
trade only one card at the same time (so no one with two accounts can trade X amount of nymphs from one account the another account where he already has a full set of one nymph)
allow a limited number of trades each week/month or maybe a limited number of trades for each account
only allow every rare card to be recieved once by trading
these "rules" might all be way to complicated to actually be done but imo it would make for a trading system with minimal abuse