A merchant or store is an object in Neverwinter Nights that contains an inventory of items available for purchase, and that can buy items from a player character. These objects are often accessed by talking to a non-player character (NPC), who is sometimes also called a "merchant", but a module may provide other means of accessing a store. (For example, the Divining Pools in the original campaign can open a store when they are used by a player.) In addition, the relation between characters and store objects is not necessarily one-to-one; a single character might provide access to multiple stores, and multiple characters might provide access to the same store.
(Of note is the separation of the store object's inventory and the NPC's inventory. The death of an NPC merchant does not result in the store object's inventory becoming "loot". Rather, that death often causes the store's inventory to become completely inaccessible.)
Purchasing and selling is accomplished from a window similar to the players' inventory; items that are to be bought (or sold) can be dragged and dropped from merchant to player inventory (or vice versa). In addition, when a store's inventory is shown to a player, double-clicking can be used to buy or sell, and the buy/sell option is part of the item's radial menu. Not all items can be sold at all stores, though. Some stores will only buy items (from a player) of a certain type, and many stores will not buy items that have been flagged as stolen. (Those that do buy stolen goods often offer less gold for them than they would had the item not been stolen.)
The exact prices offered by a store are set by the module builder as percentages of each item's base value, with a possible addition/subtraction to/from these percentages resulting from an appraise check. (If there is an appraise check, the opposed skill comes from the NPC who opened the store. This is the only time when the NPC affects purchasing and buying.) Each store has three percentages – one for selling (to players), one for the buying (from players) of normal items, and one for the buying of stolen items. The defaults for these are 150%, 35%, and 25%, respectively. In addition, a store may have a maximum price that it will offer for items bought from a player, and a store may have a limited amount of gold available. (A store that runs out of gold will no longer buy items from players. A store's gold is replenished when it makes a sale.)
Since a store's inventory may contain numerous items, it is separated into five categories (each with its own tab): "Clothing and Armor", "Weapons", "Scrolls and Potions", "Magic Items", and "Miscellaneous". If this division still results in more than one page of items in a category, the left and right arrows on either side of that tab's title may be used to navigate through the pages. Initially, the inventory window opens to the first of these categories that has an item for sale (or to "Clothing and Armor" if the store's inventory is empty).
Stores may offer to identify unknown items. The price for this service is set by the module builder; the default price is 100 gp. If a store offers this service, examining an unidentified object while the store is open will enable the "Identify" button in the lower left of the panel and text noting the cost of identification will conclude the item's description.