Honestly the only way to get rid of gil sellers is to devalue it in-game. As long as people are lazy and not willing to farm, there will always be people willing to spend real currency for digital currency regardless of the game.

Out of genuine curiosity I checked several gil selling websites and they're already going cut throat. After refreshing at a casual basis I saw the prices of a few websites undercut one another by pennies at a time until another site undercut by almost $5 before it stopped entirely.