Scattered throughout RuneScape there are many shops where you can buy and sell items. Most the shops will specialise in a certain type of item. For instance the sword shop will only buy and sell swords. In addition to the specialist shops there are several "General stores" which will buy and sell almost anything. However items in the general stores normally cost sligtly more, and they will pay less for items you sell them. If you take the trouble to find the right shop you can often get a better deal.
To access the shop you must talk to the shop-keeper in the store and choose the conversation option to indicate you wish to buy or sell. A screen like the one shown below will appear.
Next to each item there are two numbers. The green number indicates the number the shop has in stock and available for purchase. The blue number indicates the number you are holding, which you could sell back to the shop.
Click on an item with the left mouse button. The item will be highlighted, and the purchase and sale prices will be indicated at the bottom of the screen. These two prices will not be the same since the shop buys at a lower price than it sells out (the shopkeeper is making a hefty profit!). You can then choose to buy or sell the item by clicking on the relevant buttons.
The price of items in the shop also depends on the current stock. If a shop has 500 swords in store it will sell them at a reduced rate to try and clear stock, however it will also pay less for them if you are selling.