These merchants sell the Raid Vendor Armor Sets. These gear sets are designed specifically to help you progress through the end game raid experience. The sets for each faction have very different visuals even though most of the sets names seem similar.
The Defiants have only one Raid Armor Set vendor, Lepatria Freetairn. She can be found in the Catari Command Center in the right underground wing of Meridian, around 6104, 913, 5081.
The Guardian Raid Armor Set Vendors can be found in the Lodge of the Plains, around 7594, 869, 3100.