Yeah I've played on and off since 2004, and I've seen the prices of items fluctuate directly as a result of the gil seller vs the GM moderating activity. More gil selling, prices go up, more GM activity prices crash. I'm guessing this is due to the gil sellers that specialize in crafting, that they want to get their craft of specialty up to maximum level as soon as possible so they are willing to buy materials at the auction house for more, which creates a supply-and-demand bottleneck.
(For example; I remember when I was starting out clothcrafting in 2004 and how hard it was to get cotton to level on (I did not know a the time about harvesting or taking it from mandragoras with a thief job). It would be so frusterating as a level 30 white mage to get through the level stage where cotton was required, so buying cotton was very expensive.)
But it seems surprizing to me that they don't have a tagging system in place with the automated system that can trace the origins of gold. It shouldn't be too hard to notice that gold is coming from the sale of ore grown by a character attached to the same account. Strange.