As you gain more levels in different skills to do or collect better items, or make things for killing other players in combat, the economy is based on the buying and selling game items from NPC’s or players. Among the latter, you can barter with items or money with an interaction within the game. Some limitations include prohibitions to change things about holiday events (Christmas, Halloween, etc..) Or quests, and a limit on the amount (ranging from 3,000 to 60,000 gp) of what can be uneven as barter value. Alternatively, many more as players use what is known as the ‘Grand Exchange’, an establishment in the city of Varrock where prices are controlled by supply and demand. A player may offer an item or buying one, giving some value to what you want (with a limit of 5% higher or lower than market price) and put on sale. You do not need any direct interaction of another player to get what he wants. Just wait until the offer is completed to withdraw your winnings.
To control the game economy, prices change by demand. If players purchase items at a lower price than the market, the value of it drops, if they sell items at a higher price than the market, the value rises.

Runescape private servers guide gaming

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Source: Runescape Private Server