Since Gil buying and selling seems to be ok to Square Enix, I'm not sure this will be heard by someone who can actually make a difference. Tried in-game reporting several times, only to be met by a canned response, and no changes at all.
So, here are some ideas to help deal with RMT:
1) There's only selling because there's demand for it. As soon as people start having characters either perma-banned or seriously punished, and word spreads, people will think twice before buying. They only buy because they are 100% sure nothing ever happens.
2) Since the blacklist fills from 0 to 200 in a couple days - in Ultros, I get a spam message every few minutes, then just get rid of the 200 limit.
3) Along with #2 above, why not have some sort of semi-automated system for banning? Like... if Mr. Asdasd Goldahceo, a lv.1 pugilist is suddenly blacklisted by 500 people, there's a very good chance they are just bots. Ban them temporarily, and send for review by a GM.
4) Throttle /tells. Normally, players don't send/receive tells from lots of people in a short amount of time. I do communicate by /tells frequently, but mostly with 1 or 2 people. Throttling would make spamming much slower: like, you can only send /tells to 3 different people per minute, or so. Adjust this so there's few false positives.
5) From my full blacklist, I can see after a few weeks that there are perhaps 5 or 6 (Deleted). So, account deletion isn't being treated seriously, I guess.
6) Make RMT reporting easier, as adding to BL is. Right click >> Report RMT Spam. Having to fill a long form for it is not nice. And after reporting, seeing the same spammer there after 2 weeks discourages future reporting.
7) Some RMT spam I receive claim to send gil via in-game mailing system. I guess they use Moogle delivery? If so, messages with large amount of gil could be screened by some semi-automated process. It's ok for a lv.50 everything friend of mine to send me 5M gil for something I did. It's probably not ok for some half-leveled random person I just added to send me 5M gil. There could be manual screening as well.
8) Actually do something when reports are filed. I've met and reported people who doesn't have any crafting classes leveled (so, can't earn gil from crafting), who haven't cleared T5 yet (so couldn't earn gil by selling runs), and still have bought a 100M house, with furniture exceeding 150M gil. Where did that much gil come from? Yet, he's still there, fully geared, melded, and so on.
8.1) Met another guy who had clearly bought gil, and he admitted doing so, because he started playing recently and had to catch up somehow. Reported again, and nothing happens.
9) "Legit RMT" seems in fact to be ok with SE. By legit, I mean buying items in MogStation just for selling in-game. Met people who bought tens of marriage stuff, selling in-game. Made like 40M gil. I don't know about you, but RMT is "Real Money Trading", so, Real Money is Real Money, no? If this is ok for SE, then why not just start selling "gil packs" ?
10) Isn't this illegal in some places? Why not start filing lawsuits where they are? The game economics and ecosystem should be top priority.
Going for the sellers won't work, if the buyers continue buying without anything ever happening to them. And believe me, they are completely confident nothing will ever happen.
I'm not claiming RMT solutions are simple, but despite the "actions taken against rmt" post we see every now and then, what SE says they do and what we see in-game are completely different. Have they tried just staying logged in to see the utter mess the spam situation is nowadays?
That's it for now. The game is awesome, but dealing with spam every minute, and seeing people cheating all the time is just very hard to take.