Mostly gold sellers. A majority of them have trading licenses, and they plant themselves at various locations throughout the channels to make it seem like the price they are setting for items is the standard price.
A ton of gold buyers also know their customers buy gold as well, so they try to get the most gold from their fellow buyers by attempting to match the prices. If a regular player comes along, notices a price, then lowers their item a bit to try to get the item sold, the gold sellers and buyers come along and buy it then put it up in their shop for a higher price. Once all the low-priced items are gone a new default price is made.
A lot of the gold buyers end up buying from gold sellers too out of desperation so the gold sellers are laughing at their customers while their thousands of employees play 4 chars at once farming items like mad and hacking pots with the pot hack. It's a cycle.
There's also that "US Server Greed" that goes on due to American servers being so behind compared to their Japanese/Korean counterparts, cash shop wise and content wise. Random people going around mentioning that when x update comes, y will become useless so you'd best be trying to make profits off of it by selling high high high. Or that on JP/KR server this item goes for x millions because it's really good, so you should price it as such on US server even if no one has the gold to afford it. A " OMG R@RE OMG = $5000000" thing.
I'd say eventually things will mellow out and go to "normal", but the game is too easy to hack in many ways and the protection is pretty lame. Even Korea has pretty much abandoned 2 of their 10 or so servers and just let it run rampant with hackers and gold sellers.
Even when the auction house comes and it lets everyone sell items on it (or pay $ to sell even more items on it), it won't change a thing unless Nexon can defeat the gold sellers and such.
...like that's going to happen.