Now as you probably already don't know, I am an avid Runescape player, and have been studying it for quite some time. I would like to bring you into my realm of my studies.
Well to start off with, they made a majority of trade between players not actaully be face to face. Like, you would want to buy this item, and you set your price(there is a range for each item) and you would either have your offer in, waiting for someone to sell, or buy it from another player without interaction. This little device is called the Grand Exchange, which looks and sounds surprisingly like the stock exchange, and pretty much is.
Now, prices rise and fall depending on the average price the item was purchased that day. Say at item has a limit of 5 and 12. The "market price" would be 8 or 9. If a lot of an item is bought at more than 9 coins, the market price will rise the next day. The limit will now be 9 and 15, and the market price will be 11 or 12.
Unfortunately, this system is fairly easy to abuse, and a group of rich players can all buy one of an item at the maximum price for a few weeks, and it will rise dramatically then fall dramatically. I call this an unnatural rise.
Example of a rise
Example of a rise and fall
This is horrible. And hear me out. When an item is being "bought out", it makes the item impossible to buy for a few days or even a week or more. And when it is sold or "dumped" it's impossible to sell for several days.
Now this is my job. When the game updates its content, which it does fairly often, I like to find the connection to an item and prepare for it to rise or fall. Recently they changed the herblore(making potions) skill, so that there are new higher level potions to be made. The potions require and herb to make, and which can be farmed. Of course people want to level as fast as possible so I bought seeds that make the herbs and also the herbs themselves. Investment I am still holding on to :).
Now some items like these herbs and seeds rise on their own or a natural rise.
Here is an example
It's very fun to how these things work and its sometimes hard to find that connection from the update to the item.
Hope you enjoyed.