Lately, the amount of people disgruntled with the state of the economy seems to be on the rise. The vast majority like things just the way they have always been, and are little helped by price fluctuations. Stability seems to be the watchword.
Here's an example: recently we saw an odd spike in the value of Maple Logs. Normally they are stuck on their bottom fixed price of 36gp, but for a brief moment they jumped to 39, even instantly selling at 40 for a bit. The reason they normally are on their lowest possible value is that thousands of Maple Logs are basically a by-product of the Miscellanian Kingdom.
However, people do use them, and on occasion in large numbers. Cheap Fletching and Firemaking come to mind, but these logs also serve their purpose in junk-trades. But theoretically, none of these can have a significant impact on the price of this item. Let's face it, the Inferno Adze has been made available to us quite a while ago.
I'll explain. Maple Logs are a tradable item that falls under the massively-supplied-but-hardly-needed category, kind of like Fire Runes. Thousands of people continuing to use their Kingdom can potentially dump millions of Maple Logs on the market every single day. And unlike, for example Coal or Flax, which can be used to make profits, there is no high demand for Maple Logs, other than training Firemaking cheaply. Another example of an item in this category is the Blood Rune, as they too are bottomed out, with an occasional spike.
So what can cause these spikes? For that we have to examine what factors can truly have an effect on items traded by the millions every single day.
The most obvious ones are game updates. For example, we just had an update that would no longer allow boss and minigame specific drops on PVP and Bounty worlds. As a response to the item being supplied less than before, the price of a Dragonfire Shield rose a good 800k on that day.
Another force that can have a substantial impact, or at least a temporary one, is the phenomenon that has quickly turned into the major corporations of RuneScape: merchant clans. If you know what to look for, you can easily recognize their hand in certain price fluctuations.
A J-mod on behalf of Jagex has acknowledged that price manipulating merchant clans are a problem, and that they are looking for a solution to deal with them. But how can you? A person can still choose to spend all his cash on a stockpile of items, so can a thousand people doing that in an organized fashion. There are no rules yet that would treat a collective differently than a number of individuals, so if Jagex decides to move against collective activities, it'll be a first.
The last major factor is a known one: inflation. There is more and more GP created now than there is removed from the game, and this is normal. With ever more people playing at once, you need more GP circulating, or people simply wont have the cash to pay for their goods. The problem right now is that there is vastly more GP brought into circulation than needed to keep prices stable, and with a higher average of GP per person, prices have the room to go up. So far, Jagex has not implemented a "tax rate" (think along the lines of banks proportionally charging you for storing items), which is one of the most used tools in the real world to combat inflation.
The rest of the price fluctuations boil down to simple changes in supply and demand. However, sometimes on a rare occasion, someone finds a new use or something else previously unknown that can cause a run on certain items. There are a few other minor impacts, such as the holidays, and the GE limits. So there they are, the reasons why the value of your items can jump.
Obviously, there is little you can do to control this. Updates come and go, merchant clans are still legal, probably will be for a while too, and inflation was stopped only temporarily when Construction came in. So my advice to the common man who just wants a bit of stability is simple: never buy for more, or sell for less than you want to, and if the price range is out of yours, move on to the next item.