Dungeon merchants are vendors that require tokens acquired from a dungeon in exchange for goods. Like merchants, these NPCs have a "[Merchant]" suffix after their name and a normal merchant icon. Given the current reconstruction of Lion's Arch, the dungeon merchants are now at Farshore Ward and have a buyback feature.
List of dungeon merchants
Lion's Arch (post-assault; under construction)
Lion's Arch (pre-Assault on Lion's Arch)
Vigil Keep (temporarily during the Assault on Lion's Arch)