Pepsi, It seems you misunderstand how the system SamRF is talking about would work.
Player A would pay SE a full-price month subscription to obtain a token. This cost would be in addition to the costs they normally pay SE to play the game.
This additional token, which SE has already received a month subscription's full price for, could then be traded to other players for market-fluctuating values of gil.
Tokens would not be generated for free out of gil. SE would still get the money first.
The RMT wouldn't be buying subscription time tokens and selling them; that would be a poor decision because they'd first have to pay SE money, and then they'd have to sell it for less than they gave SE because why would anyone buy sub time from a third party for more than what SE themselves sell it for?