I am afraid to address the economics of the trade in too much detail because it's hard to tell at what point discussion about RMTs crosses the line of an overzealous ban-hammer, but, on my server, it is far more cost effective to trade wristlets to real players to generate gil than to buy it from a shady RMT, or to buy your wristlets from SE directly if you're the sort who's willing to part with real money in the first place.