Ever since the introduction of the Grand Exchange there has been items where demand has surpassed supply by great margins. These items are quickly bought up by merchants and investors alike; thus being turned into unbuyables. Unbuyables, as the majority calls them, are items where the Grand Exchange cannot update fast enough to suit the street price of the item. Due to the fact that these items are already popular high level items and they have an increased demand from being unbuyable their street prices will become inflated from a false demand.
The first items to become unbuyable after the release of the Grand Exchange was 3rd age. Due to the extremely low GE price and the high demand for 3rd age (after the partyhat crashes) it had the makings of a great unbuyable item. This coupled with the fact that 3rd age took a relatively large amount of it to be sold to cause a price update made it the idle unbuyable item to have. Shortly after Jagex caught sight of this problem in the GE they changed some of the mechanics for high level items; most notably the amount needed to sell for an update. This caused 3rd age to update more frequently, which would make it so the buyout wouldn't last as long because the GE price would slowly reach a price where public demand would fall and cause the item to collapse. On April 3rd, 2009 3rd age fell from the status of unbuyable item to crashing junk; this paved the way for party hats to rise to the top.
After staking was taken away, partyhats suffered a massive crash because of the stakers quitting and selling their hats off. Due to street prices being much lower than the Grand Exchange on release they were unsellable. In order to preserve people's wealth, Jagex made it so party hats update 1% instead of 5% like normal items. This combined with the release of summoning made it so extra money wasn't around for people to spend on the paper hats. However, after 3rd age was dumped by merchants and pvp had been out for a while, money was available to spend on superfluous items. March 2009 was the beginning of the party hat rise to the pedestal of unbuyable items. Even with party hats having a 1% rise/fall each update they were picked up as the new unbuyable due to their relatively low supply in the public. This coupled with them being a popular item for people to work to made them good enough to pick up as the next unbuyable, even with spirit shields already being bought out.
September 15, 2008 brought the introduction of spirit shields and a new item for merchants to grab hold of. Instantly the Divine and Elysian spirit shields were shot up to over one billion in street price. The arcane and spectral were priced near accurately and were quickly stabilized in the market. However, due to the limited supply of spirit shields in the economy they were not a massive hit as 3rd age was at this time. Even today spirit shields do not have as much of an impact on the market as they have potential for because the people who get them tend to keep them for their own personal use.
Unbuyable items are created from a multitude of reasons; the two most important ones are being the basic system that the GE is based off of and the availability of GP in the economy. Party hats having been constantly rising for over 100 days now and many are thinking when will these crash? Party hats remain popular due to the inflation that has occurred from Runescape because of PvP and the lack of a new money drain in the game. Once a new money drain is introduced party hats will most likely take a big hit, if not crash for awhile to come. Spirit shields are another topic entirely as the Divine and Elysian remain to be the two most wanted shields in the game and will most likely stay strong even if another skill like summoning is introduced. Third has now since rebounded from it's crash in April and can be used as an example of what will become of any future unbuyable item in the market. Unbuyable items as evil as they may seem are used to keep the economy balanced by creating items that can be used to buy more unbuyable items. It's an entire market on it's own and once your involved in it, it's very hard to get out of , because of the items that you can achieve. The unbuyable market is not hard to get into; it just requires a bit of luck or knowledge of what will be the next big thing.