Well, it does illustrate the rule of supply and demand. When there are more on the market, one guy is going to bring down his price so the people will buy his product instead, leaving the guy who put the price high without anything. That isn't being greedy, being greedy is pricing things several times their actual amount and knowing that they did it. Underpricing things as such is an economical move to get some money while the other guy is charging higher for the same item. Not only is that good, both the person charging lower for the item is making money, but the price for the said object goes down, thus allowing people who aren't obscenely rich to buy it. I usually buy from auction, and I do see massive price fluxuations.
Example..Item X is selling for 5,000 gil. Player Y has 5,000 gil. Player Y places a bid on it for 5,000 gil. Player Y fails, because the asking price is now 10,000 gil.
So, there is a large amount of greed present in the AH. Such as what Prssha described (I have not noticed this myself, so I'll take his/her word for it.)