To find a hidden merchant in the Nechrohol of Nabudis, do the following. Head to the northwestern-most corner of the Hall of Effulgent Light. The room on the map will have a dead end, but it's not. In the secret room adjacent, you can fight a powerful enemy (you should be at least level 40 to attempt this). The southeast corner of this room is where the real point of interest is, however. A merchant will be there that can sell you rare items.