Flipping

Flipping involves buying an item for as little as possible and then selling for as much as possible within a few hours. Flipping is unique in that it can be used to profit from any item, regardless of its price trend. Flipping is only recommended for players with plenty of starting cash (10M at least) to do it, as not doing so will result in low profits.
Remember that it can be difficult to buy an item for below guide price and sell for above guide price, unless the price actually changes from falling to rising, because prices are virtually always either falling or rising, and often only switch every few days.
To determine the buy/sell range, buy one item at considerably above the market price. The amount it buys for is the high price (the highest price the item can be sold for). Now sell that one item at well below the market price. The amount it sells for is the low price (the lowest price the item can be bought for). If

An example of flipping. The item (right) is bought for less than it is sold (left).

the item cannot be bought or sold at a significant margin, it is unsuitable for flipping. If you buy an item and find that a major crash has started and you cannot sell at a high price, you can either hang on to it until the price rises again, or dump it at a low price. If you choose your items carefully, this shouldn't happen very often. A wise rule is to always do a test buy of a small quantity, and then resell to see how quickly the item moves. If you can buy ten easily, then sell ten easily, next time try to buy 50/sell 50, then try to buy 250/sell 250. Never start out buying a huge quantity of an untested item.
You also want to find items that are in your price range. If you have only 100,000 coins to invest on an item, you probably don't want to pick an item that is 50,000 coins as you would only be able to buy 2 of them. Instead pick items that are about 250-500 coins, that way you could buy up to 200-400 of them. Quantity is usually more important than price. The key is profit per hour. If you can move more items quicker, you might make more money even though it's a smaller profit on each individual item. Ever wonder why stores have big sales? This is why.
Another important concept is diversification. Do not put all your money on one item. If it drops in price, you are in trouble. Try to pick four to six good items (you may have to experiment to find four good ones) and just keep buying low, and selling high. Remember there are always impatient players who get rare items as a drop, such as corrupt dragon items, and they need cash fast, so they will sell for a low price. Then later will come the impatient buyer, who has to have that item right away and is willing to pay a lot of money. Patience is one of the keys to making money in trading.