I can't be 100% sure, but I find it extremely difficult to believe that certain people on my server are not buying gil. While on the market boards I will see the same person buy 12+ Ehcathl Sealants at 339,000gil a piece and then turn around and buy the new crafted rogue/ninja gear for 4mil a piece (2 or 3 pieces bought) So, that's roughly....
4,068,000 sealants
+12,000,000 arachne gear of Scouting
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=16,068,000gil spent in less than 3-4 days. (lord know what else they've bought, I'm not exactly tracking them, just notice their names in the history ALOT)
Either these guys are very good at farming money or playing the market or they are buying gil en masse. I cant prove or disprove it, but it does seem fishy.