I don't see why they don't just move the gimped, weird tax system currently in place, and make it so that you're taxed when posting an item. This would render the 1gil undercutting useless, as you'd quickly devalue your own item from posting it over and over, which is much preferable to crashing the market.

5% fee for listing your item, helps with people stalling the market with ridiculous over pricing as well, also requires RMT gil buyers to have 5% of the Gil they're looking to buy before posting an item at 500,000% markup.