For a more complete discussion about that economy, refer to the Economy Guide. This article deals with the more limited question of how to relate the various pricing levels with the player's many buying, selling and bartering opportunities.
There are several ways and places in which players can buy, sell and barter goods. Where governed by the game itself, prices will be predictable, or even fixed. At the opposite extreme, when players are directly interacting the prices will be less predictable, and sometimes even irrational.
Note that ordering prices in this way intermixes prices in which the player is buying or selling.
The highest price a player will typically see will be the selling price at the appropriate store. For some items, that will be the General Store, while in other cases this price is set in a specialty shop catering to that item type. General store items (for example, buckets) might occasionally show up in a specialist store, but the price and its behaviour will remain the same. Specialty shop items can be sold to a General Store as well, but it will sell for a lower price.
The Store Price is listed in every item's infobox. This price will be at its highest when the store has precisely one item of this kind in stock. The price might drop if the quantity in stock is higher: whether it drops, how quickly it drops, and how many items must be in stock before it drops will depend on the item. Moreover, the store price might well be lower than the street price if the latter is affected by scarcity. It is worth noting that the Store Price is set within the game itself, and only crudely simulates supply and demand, whereas the street price is set by humans buying and selling (and, indirectly, by bartering) and can fluctuate erratically.
The next highest price will typically be the Street Price. This should vary somewhere between the store price and High Alchemy price if the trading players are acting rationally. The store price should place a ceiling on the street price because a buyer is unlikely to pay a substantially higher street price unless the item is so scarce in stores that the price is irrelevant, such as the Abyssal whip and the Dragon platebody.
At the same time, a seller capable of the high alchemy spell has no reason to drop below that price when selling player-to-player — unless, of course, the value of the item is so low that the expenditure of the required runes makes the spell uneconomical. For the same reason, the alchemy prices might not even be relevant and thus provide no floor below the street price. That, in turn, means that players will seldom find the effort to find buyers worth the effort. Thus the curious problem that it is often easier to find a player selling a high value item than a lower value item. Unless you are selling something worthless (examples: Bucket,or Ashes) you are more likely to get a good price on the streets if you use forums. But it will generally take longer to sell because sometimes no one wants it. So sell on the streets if you don't care about getting a different price every time or waiting.
Street prices are only roughly known, since there is no formal exchange in which these trades could be tabulated. The least formal exchanges are those that occur in or near banks or town squares in any world; somewhat more organized are those trades taking place in specially designated worlds; and trades that originate in external forums are the easiest to examine and compare.
Prices at the Grand Exchange are usually around market price. However, players are only allowed to buy/sell for 5% above or below the pre-determined Grand Exchange market price. This market price is adjusted daily according to supply and demand (based on player offers in the Grand Exchange).
A recent update increased the price margin for some items on the exchange, to as much as +/- 5%.
Newly introduced items are allowed a much greater daily variance, some perhaps as much as +/- 20%.
Information about prices can be found on the Grand Exchange Market Watch
The "high alch" price is that of which the appropriate store would purchase the item from the player if the store had none of that item in stock. In other words, the highest price at which the game itself would buy an item from a player. There are, however, exceptions: the specialist shop Grum's Gold Exchange in Port Sarim will buy gold jewellery at a higher price than high alchemy if none are in stock. The existence of the high alchemy spell also explains why many mid-range items are seldom traded player-to-player: they are often created in such numbers that searching for buyers would be futile, while the item is valuable enough to make the spell a worthwhile expense.
The associated "low alch" spell exchanges the item for the price that would be paid by a General Store, 2/3 of the price that High Alchemy would provide, but this can change depending on the initial value of the item. High Alchemy is used to level mage, most people find it easy to buy yew longbows and convert them to coins using alchemy for a tiny loss.
The worst case scenario — and the only one available to new players — is selling items directly to stores. In the best of circumstances, the price will be the same (or, as noted, higher) than high alchemy prices. But as a store's stock quickly rises, the price quickly falls, and often the location of an appropriate specialty shop is unknown or not worth reaching. It is also commonly used to dispose of bulk items which are too cheap to cover the expense of the runes required for High Alchemy. A common example includes unstrung Willow and Maple bows.