trading gil is as easy as listing a random gear on mannequin for X gil and the buyer just need to buy it directly from the mannequin, no tax for either buyer & seller.
charging higher tax still doesnt solve the cross world selling as they just trade items to that world's alt to sell on the invaded world.
yeah its true that 100mil feels like 1bil relative to gear & mats, but house prices (player to player) remain the same.