How do you get really good items on runescape?

Each item has a market price (this is the average price of sales during the day or so before the last GE update). You can buy at the market price, or you can usually go up and down up to 5%. Quite often, if you offer the maximum price - or at least somewhat more than the market price - you get the item you desire faster. If it's not too much money for you, try it. Some items, however, are already at their maximum price (the GE limit), in which case there isn't much you can do.

Similarly, when you want to sell an item, it will sell faster if you specify the minimum price (usually about 5% below the market price). Here, too, there can be a price minimum. If (for example) you want to sell fire runes or mind runes, you may have to wait a long, long time, because the market is quite saturated - that is, way too many people want to sell fire runes, in much greater quantities than what is really needed.