Ultros seems somewhat susceptible to this problem, some of us were having a discussion about it, the ban shout until x lvl idea was brought up, but I personally feel it's only treating the symptoms and not the disease. If it's becoming such a wide-spread problem across all sectors of the game, just put into place a system similar to Gaia online, where you have to use a specified NPC to trade, this way accounts that trade extreme amounts of gil can be identified and investigated.

If that fails to deter them, ban trading gil between players period, while I know it's a case of the cure being worse than the disease, what would you rather put up with?