Are they easy to find and notice? Do they show up as merchants on the map?
They're called Quartermaster (they're the guys who sell siege equipment, in addition to handling the guild stores). They spawn inside a keep once it switches hands, and remain until it switches again. The exact location varies by keep, but it's usually near the Transitus Shrine (sometimes in the same room, sometimes in the room across the hall, etc.).
There are also Quartermasters at resource nodes (farms, mines, lumber mills). They stand outdoors, usually by some little building across from the node's tower.
They're easy to spot once you find them, but I don't remember whether or not there's an icon for them on the map.