This Rs guide to merchanting in Runescape is to give everyone a chance to earn some cash by buying and selling items. Merchanting is the act of buying items for a low price, holding on to them for a little while, and then selling them for a higher price once the item's price rises.
When you merchant an item, you are trying to get the demand for the item to rise, making the price raise up in the Grand Exchange as people try to buy the item. To do this, you need to be able to take a significant amount of the item out of market making demand rise above supply, driving the price up. Getting the supply of the item out of market is really only possible by joining a merchanting clan, because there has to be such a large amount of the item taken out of the market.
If you don't want to join a merchanting clan or can't get into one, you are probably going to want to do long-term investment merchanting, where you will buy an item for low and sell for high as the market naturally rises and falls.
There are three things you want to look for when picking an item to merchant:
1) Value - The more the item is worth, the more it will be able to fluctuate in price. Usually, people merchant items that are fairly expensive. Many items range from 500k to 3 million gp; however, they can get up to 50-75 million gp or higher.
2) Rareness - If an item is rare to get in a drop from a monster or in pvp worlds, it can often raise your chance on being able to merchant it. For example, an Amulet of Fury is a very good item to merchant. Not many players in the game can get a Fury as a drop, they average around 2.2 million gp, and they are used quite frequently so there is a demand for the item.
3) Demand - For an item to be able to increase in price, there has to be a high demand. If you are in a merchanting clan and you all decide to merchant Amulets of Fury, then after you buy many of the amulets out of the market and hold on to them, supply of the amulets goes down, while demand will stay the same or even increase, which is what drives the price up.