Square Enix is a Japanese company. IGE is American, and the majority of Gilsellers are Chinese. Transactions are done via internet and are therefore nearly immune to any form of legal or corporate enforcement. To take any legal action, SE would have to have government approval of both Japan and whatever nation the other party is located, America or China. The only thing that SE can do, is try to find gilsellers and buyers and delete their accounts, which is hard to do, since they only can see the gil being either sent by Delivery Box, or by trade.
Now, the methods of gilsellers use to obtain gil aren't hard to spot, but having the actual proof of gilselling and buying is. I've often sent large and delivered large amounts of gil, I could be a gilseller myself, right? But I'm not, but for all SE is concerned, me sending 100k gil to a person in my LS is an act of gil selling/buying. It's hard to know for sure.
Gilfarming isn't at all wrong, or illegal by the Terms of Agreement. In fact, nearly 100% of all players have 'farmed' for gil at one point or another. Be it camping an NM, or farming Beehive chips, doing BCNMs, or quests. So you can't just lable a person or group gilsellers based on the fact they've been in Batalia Downs for 10hours killing all the tigers. You need to catch them in the act of actual buying and selling. Again, that's hard to do, SE can't obtain (legally anyways) the receipts of transactions between players and companies.
To make matters worse, GMs seem inept at ending other ways that gilsellers get gil, which by the TOA, is illegal, such as MPKing. I've sent two messages to a Game Master about the Wangcai team, for MPKing me alone. I'm not the only one, I've talked to various other players who have also been MPKed by them, and sent messages to GMs. Common sense would dictate that after so many calls about a certain group performing actions like this, you'd think it would be okay to at least imprison their characters for a bit, if only for a couple days.
I realize that many gilsellers are literally doing this as a business, and make a living by doing this. In China, the money you can get from this, is far greater than you would get from doing 'normal' work. But by the sheer stupidity of this, they've ruined their own form of living. I don't know how it used to be, but I've been told a Million gil used to fetch something in the $300+ range, now, you can get a million gil for $60. That's 1/5 of what it used to be. Can gilselling still be so profitable for them? At a certain point, the time invested in this will no longer be supported by the amount of money you make.
Here's what SE needs to do. I'm almost positive somebody has brought this up, but here goes. Have SE set aside both a character for buying and selling a small amount of gil. Have one character buy only a small amount from IGE, last I saw that had a 10dollar amount. Once they recieve that gil, instantly delete the senders account. No questions asked. I'm sure he/she would have more gil than just that, so you're preventing more of this '*******' gil from entering circulation. Now, I'm not entirely sure how the selling of gil to IGE works, but I think it'd go something like you notify them, then they give you the character name of to either sell it too, such as a designated gilseller, or a normal gilbuyer. Whichever, once you see those names, instantly delete those accounts, no questions asked.
By doing it instantly, they can't quick send the gil to some form of backup account, all that gil is terminated. Do this in one quick sting operation, and in a day you'll have killed off a large number of gilsellers and buyers. Also, with the constant threat of a SE sting agent, gilselling and buying would be rather risky, and I can' think of many players who would risk 6 months over a 100k gil.