Directly from the ToS:

2.2 Real Money Trading, Farming and Power-Leveling. You may not sell, purchase or exchange for real-world money or value any in-game currency, accounts, characters, in-game services, or in-game virtual items. You may not play the Game for the purpose of acquiring virtual items or advancement in game play on behalf of a third-party or for the purpose of selling any virtual assets to a third party for real-world money, specifically including "gold-farming" and power-leveling services.
source: http://support.na.square-enix.com/ru...2&tag=users_en

Of interest is the following snippet:
You may not sell, purchase or exchange for real-world ... value any in-game currency ... .
Value in this context implies monetary worth. Art has value, and as such the trade between art and gil would conflict with the ToS. So in essence by selling your art, you're purchasing gil. And the person purchasing the art, is merely selling gil.