Vendor

Vendor, merchant, blacksmith and other related words all refer to a NPC who is selling and/or buying and/or repairing goods and/or services. If this escaped you, it's a pretty broad term for the sort of "useful" NPCs that are (mostly) not directly involved with quests and story progression.

Background

Since the beginning of RPGs, there has always been someone there who was interested in buying your "Rotten Apples" or "Diamond Rings", and also able to sell you new treasure for gold, or whatever the local currency would be.

These can be called by some generic term like merchant or vendor, but is sometimes referred to with their real profession, like a blacksmith. Depending on title, the items they sell can vary, like a potion master would only sell potions.

Some vendor NPCs are able to repair equipment and other functions, and while they mostly are not involved in quests or have proper dialogues, they sometimes do that as well. The terms just refer to the game mechanic of selling and buying goods and services to a non-player character, and does in general not limit other functionalities like being a quest NPC or vital to the story line.