Is buying gil legitimate

Buying gil from one of those advertisers is not legitimate, and if you do buy and SE manages to track the transactions back to your account, they can and will ban you from the game. Not to mention what could happen if the gilsellers manage to get your password or personal info. There are easy enough ways to make gil, I have a decent amount right now just by having leveled a gatherer class and doing some low level crafting, and by farming around for drops that sell fairly well for a good price. Don't ever buy it from a third party. Those guys are operating against the game's terms of service, and they are not your friends, no matter how good they make their offers sound.