Keefdy Marnhedge is one of the Dungeon Armor Set Vendors for Guardians. He can be found in the in the Lodge of the Plains in Sanctum, around 7594, 869, 3100.
Keefdy Marnhedge sells Tier 2 expert dungeon gear which can be purchased using Plaques of Achievement. These plaques are earned by killing bosses inside expert dungeons and completing dungeon daily quests.
This is only a listing of the armor he carries that has a visible model. He may carry more items such as necklaces, belts, or weapons, but they are not listed here.