A vendor is an NPC who buys and sells items. Trainers are not technically vendors, even though they sell recipes, since they do not buy anything. The word is also sometimes used as a verb for selling to a vendor, e.g., "I'm just going to vendor this axe, since I can't use it." For quick use as a verb, sometimes only "V" is used to mean "vendor it" or "V?" for "vendor it?" Items that have no use other than selling to a vendor are called vendor trash.
It is noteworthy that while most profession and food vendors are essential, most armor and weapon merchants are only as valuable as their repairs. The items they sell are expensive, have no stats, and are almost always completely inferior to items of the same level. They have little to no use after around level 10, when drops and quest rewards become better.
interact with vendorvendor out of rangebuy/sell after talking
Some vendors can repair armor durability damage (usually vendors who sell armor and weapons, but often those who sell blacksmithing and engineering supplies).
As of Patch 2.3, your pointer changes when you move it over a repair-capable NPC:
interact with repairing vendorrepairing vendor out of range
If you sold an item, you can buy it back for the same price at which it was sold by clicking the Buyback tab and right-clicking the item or dragging it back to your bag. You can repurchase up to 12 items that you have sold to a vendor. The items (up to 12) since you last logged in will persist. When your character leaves the game or zones to a different area, this list of items will be cleared, so if you have accidentally sold something, don't wait to buy it back. To this date, no raid is yet Vendor proof. Buy it back immediately.
A few items may be used as portable vendors; players can bring them with them as items or abilities to access a vendor anywhere in the game. This is very useful as it saves a lot of time to not have to return to a stationary vendor every time you have to sell and in some cases buy something. Examples of portable vendors are:
Notice that portable vendors, while most sought for as convenient buyers of useless things that players don't need, often offer a somewhat meager choice of items that players may buy for self-usage, such as: materials used in various professions; food; and, in rare cases, equipment. Most portable vendors can also repair, thus they are often popular in raids or instances.