Instructions
Identify an item in the Grand Exchange, low enough in quantity and cheap enough to be affordable enough to buy all of that item. Even for a small buyout, this may be as much as 20 million gold.
Wait until another merchant or clan buys all or most of the particular item, then watch for the moment when they dump an item. Dumping is done solely to manipulate the price of an item, flooding the world with far more of a commodity than in demand. You need to move quickly because the clan has the same plan in mind to buy back all of that particular item at a rock bottom price. This essentially uses another merchant's investment strategy against them, taking advantage of the commodity collapse they caused.
Purchase the commodity at the cheapest price available, in as large a quantity as possible. Items in the Grand Exchange have limits on how much you can buy, but if you use a few "dummy" Runescape accounts to buy as much of an item as possible, this shouldn't be a problem. In many cases, you can not achieve a total buyout, but this is adequate since your proceeds on this trade are pure profit.
Watch for the upward tick in the commodity performance (as demand increases) and set your sale offer to the maximum allowed. As soon as there is a downtick, stop selling until you see another uptick, which might not be until the next day. Repeat this as many times as necessary until you sell your entire stock.