As you talk to NPCs and meet certain requirements, some merchants will sell rare and useful items. Here are some to watch out for. Remember that you can resurrect a dead merchant by offering souls at their gravestone.
Maughlin the Armorer: Immediately available in Majula.
Sells Alva set after 16,000 souls and will give you Aurous set if you speak with him having 0 souls in your inventory. Sells Moon Butterfly set after bonfire intensity 3 and Moon Butterfly Hat after intensity 4.
Steady Hand McDuff: Found in a cell in The Lost Bastille accessible by blowing up the wall with a rolling explosive barrel. He will sell and smith items for you once you give him the Dull Ember from the kiln lava area in Iron Keep.
You can move McDuff off the chest he is sitting on by lighting the torch in his cell. There is a Twinkling Titanite in the chest.
Straid of Olaphis: Unpetrify him up the stairs in The Lost Bastille near the bridge to Sinner's Rise.
Sells Great Lightning Spear and Sunlight Blade after finishing the Undead Crypt.
Weaponsmith Ornifex: Unpetrify the Lion Clan Warrior to your right from the Shaded Ruins bonfire and kill him to obtain the Fang Key. Use the key on the door down in the Shaded Ruins corrisive pits to free him and he will move to a room in the bottom of Brightstone Cove Tseldora.