It would start with a small group of players convincing a bunch of casual players to join their clan and buy out as much of the supply of a specific item on the market as they could. They claimed if everyone held on to the items for two weeks the price would rise and they'd all get rich.

Of course, the leaders would then sell their items one week in. This would cause the price of the item to drop, resulting in the other members of the clan panic-selling all of their items. The price of the item would then crash resulting in most players losing their money.

Eventually the game developers made the organization of such clans a bannable offense since too many casual players were losing a lot of their wealth participating in these events.

[0] https://runescapeclans.fandom.com/wiki/Merchanting_Clan