Jagex infamously describes the Grand Exchange as "RuneScape's stock market." On the surface, this is an analogy that is not quite accurate. One thinks of a stock market as a place where you trade shares of a company, whereas on RuneScape we're trading goods directly. Of course the tradeable items are all virtual, but they are tangible objects that serve a precise purpose in-game and are identical. To see this more clearly, imagine the comparison between buying shares of ABC fruit company (that's Apples, Bananas, and Couldn't think of a fruit starting with C) and purchasing fruit directly from a large orchard selling apples. In the former case, if ABC Fruit Co. sells a lot of good fruit, they make good profits and you can sell shares for more. In the latter, which is more like RuneScape-what you see is what you get—apples. And (as long as you sell them before they go bad) you could set up your own fruit stand and sell individually at a profit to passers-by.
Now imagine that a very wealthy gentleman named Bill decides that he is going to buy so many apples that there will be no supply left so he can monopolize the market and sell at whatever price he wants. With nothing to stop him he can go ahead with this venture: the prices of apples quickly skyrocket and Bill either makes his fortune or ends up with a lot of rotten apples.
How does something similar apply on RuneScape? In almost exactly the same way, actually. People (or clans, 'merchanting clans') could buy a ton of an item, temporarily shorting out demand and causing the price to fly up. Of course, eventually they will sell them, causing a resulting crash in prices. Imagine if Bill's apples were still fresh for another year and he dumped them on the market along with the next year's harvest. Yes, just like in RuneScape, for any given item, in time more will be produced. Then there would be twice as many apples that year and profits would fall.
Except that RuneScape has a mechanism to (try) to prevent such catastrophes. In case you didn't know, any major real-world stock exchange has all sorts of limits and conditions on how much you can buy and what kind of trading you can do. Even on a more local scale, I'm sure you've seen ads in the Sunday paper: 'limit 2 per customer.' Although the issue there is not resale here, I think you get the idea.
RuneScape directly limits how much of a certain item you can buy off the Grand Exchange. Let's say you want to buy tin ores. The first time you buy a tin ore, a timer is set for 4 hours. During those 4 hours, you can continue buying tin ore, but once you have bought 25,000, you cannot buy a single tin ore until the timer expires. Should the timer expire and you are still under that limit, it just resets until you buy another tin ore.
This is all well and good and, in the example of tin ore, most likely successful at preventing an individual from buying out the whole stock and single-handedly driving up prizes. But for some items, the (current) limits are simply inadequate.
You might remember that you used to be able to buy and sell virtually every armor set in the Grand Exchange—things like Saradomin armor set (lg). The upshot was that the sum of the prices of the components of said set was often substantially different from the set itself. By purchasing the components and assembling them into a set, you could make an easy profit upwards of 100K. I guess Jagex felt this was an injustice, because they removed the sets and quoted essentially that reason.
Now, suppose Bob wants to buy Saradomin armor. Bob has saved a lot of money and is excited to buy all four pieces. He isn't an expert on Grand Exchange mechanics but has a feeling if he pays slightly extra, he will get them before long. He buys a helm and platebody before long, but quickly runs into an issue. Neither the platelegs nor the kiteshield will buy, even at hundreds of thousands above market price! If you haven't guessed, yes, Bob got hit by the quantity limit. You can only buy two treasure trail items of the same 'class' (like all Saradomin-trimmed armor) per four hours. So what does Bob end up doing?
He might leave the offers in there and wait, hoping to get it eventually. When four hours is up, the G.E. will immediately match his offer against the lowest sell price and complete it, granting him the item and some change. This will likely hit some offer by someone flipping that item for profit.
The other possibility is that Bob roams the G.E. on a trade world, thinking the item is low in supply right now, and ends up buying it well over the guide price as a result.
Now, while neither of these cases are "wrong" or "injustices," it seems to directly negate Jagex's action of taking the set out of the G.E. in the first place! I would suggest bumping it to 5 or 10 treasure trail items in cases like this, or "unchaining" them, to make it possible to buy 2 kiteshields, 2 helmets, etc., but no more than two of one particular item.
Many low-level armor items like mithril or adamant have some base value. You might naively assume that there is an equilibrium price between people who smith these items for xp and those that use them (lower level players or higher levels using it as a spare). However, you would soon find that smithing adamant warhammers or what have you is such a huge loss that no one does it. Similarly, the fact that one particular weapon is vastly superior to all the others implies almost a complete lack of demand for any others.
The only real thing between the prices and the number 0 is a little spell called High Alchemy. For one nature rune, you can turn any armor into a halfway decent amount of gold. However, try it on a bar or ore, and you will recieve a trivial amount of coins. Ask any alcher about items and they will likely complain that the limits are too low (for example 100 for rune items). On the other hand, try actually selling a bulk amount of those items and you will see that you get only one or two sold every so often. Once in a while an alcher's offer will hit and you'll sell a hundred. You have the choice of selling at a price low enough to make an alcher a huge profit, or waiting forever at a higher price, if not alching yourself. If limits were raised, either profit would decrease or selling would speed up. Wouldn't that be more stability rather than less?
For one more thing, consider "low level" gems/jewellery. Sapphires, emeralds, and rubies have a buy limit of 1,000 per 4 hours. If you cut them (an expensive but fast method of crafting xp), you will go through them in a fraction of an hour. Compare this with 25,000 logs, which would take literally days to bonfire, or using up any type of ore (including tin/copper ore to make bronze), or 25,000 runes to cast combat spells. It seems something like 5,000 would be more appropriate—then again, maybe there wouldn't really be enough sellers to keep prices stable.
Quantity limits are an essential restriction that keep Grand Exchange prices somewhat under control. Like anything, people find a way around it and "merch" items, typically with a whole group of people that have enough buying power, but for the most part you can trade a reasonable quantity of your item within 5% of market price. However, for some items (due to natural changes in the game), some of them could do with some adjustment.