Merchants (also known as vendors) are the primary source of items and equipment in the world of Fallout, aside from scavenging or looting from dead enemies. Merchants operate out of either fixed stores or traveling caravans.
Merchants usually specialize in a certain area such as weapons and ammunition, food, armor, and clothing, etc. but some merchants (oftentimes scavengers or traveling merchants) carry all sorts of items.
Interactions with the player character
Some merchants offer medical aid, rent beds, and repair equipment. To discover which of these are available, the customer needs to interact with each using dialogue choices. Merchants may also give discounts to them as a result of Karma, perks, or sometimes in return for personal favors.
Effects of player's actions
Killing a merchant gives one access to the store's wares through a key or directly on the body, however, this is not always the case as often they will just have default loot.