Square has logs of the transactions. They do need to get rid of the duped gil from the market, thus people losing money. Many items were utterly overpriced and those putting the prices (crafted armor mostly) knew what they were doing. at first as people where hitting 50 , the weapons needed for the relic quest which requires just a drop from wanderers palace which is easly obtainable and 3 other easly obtainable items jacked the price to hell and back. then the lvl 70 crafted armor way over reasonable prices. Of course the desprate goes ahead and buys gil to meet these rediculous demands knowing not many would even have 1/6 of the gil demanded for the item. they shot themselves in their own feet when they decided 1.5 mil was decent enough for a crafted lvl 70 item without materia involved (back then) Now those prices have dropped considerably.