Selling gil like Warcraft does will not cause inflation.

Because the way Warcraft does it is allow people with gold to exchange their currently in-game gold for game-time tokens. The people paying with dollars are basically buying subs for people in exchange for their gold (which is already in the ecosystem). So it doesn't add to inflation.

However...

FFXIV is not Warcraft, and there's a lot of FFXIV crafters sitting at millions and billions of gil with nothing to spend it on, because gil is worthless to a point. So if that gil all of a sudden had value (game time tokens or mogstation items, the release of gil from gil-barons into the market could cause an inflation. But on the other hand. How damaging can gil be when so many people have so much of it and just hoard it anyways? Gil can't do you squat beyond a house and crafted gear.