Honestly, I'm not really sure why people are flipping out about people being able to sell promise wristlets for gil. If anything, the people who didn't want to pay actual money should be excited by the opportunity to just pay for it with gil, right?
Personally, I don't think $40 or whatever is even close to worth it, but a few million gil? You might have something there.
Would the world really be so awful if EVERYTHING in the cash shop was able to be sold for gil? I mean, the reality is that RMT already exists. People are already out there buying gil. I think that's stupid, but that doesn't mean people aren't going for it. Is the game hideously broken now? Because the same people who buy gil now would be the same people subsidizing all your cash shop purchases. Probably more people would do it, but uh, so?
Unless you're looking for world firsts, you don't have any strong NEED for much of anything you buy with gil (and if you're elite enough for that and doing stuff like getting full materia'd i110 crafted as soon as it comes out, and actually going for it, you're such a ridiculously small percentage as to not even be worth discussing in terms of broad demographics), so the economy isn't really a deal breaker. If it was, this game would be pretty awful already, RMT or not. Because the way the economy is set up is pretty awful.
Making all cash shop purchases sellable would be pretty much an unadulterated win for SE, as cash shop purchases would undoubtedly increase.
If you've got some compelling arguments on why RMT would actually be really bad in this case, and I mean beyond "omg it's bad", that'd certainly be pretty interesting. Certainly, if you're anti-Cash Shop RMT but pro-Cash Shop, I'd be really perplexed.
If anything, everything being sellable for gil would make me hate the Cash Shop less because then anything I did want, I could just pay some gil for. As opposed to them someday putting an outfit or hairstyle on there I really want and me quitting because I'm not giving them real money for it. We're already skating super close to that with putting dyes up there for a hilarious $1 apiece. Thanks, SE! You found a way to annoy me even more than with the other dyes you can only get via TOTALLY RANDOM retainer ventures and that CAN'T BE TRADED WOOO!