Follow the instructions below to unlock new items from the various merchants.
Grave Warden Agdayne – You can find him in the Undead Crypt. Once you have the King’s Ring, you can talk to him to get his armor.
Carhillion of the Fold – He is in No Man’s Wharf, but after you talk to him and defeat the Flexile Sentry, he’ll go to Majula. Once you get through the Shrine of Winter, you can buy an unlimited amount of spells from him. If your Intelligence is 30 or higher, you can get the Northern Ritual Band +1 from him, as well.
Stone Trader Chloanne – She is initially in Harvest Valley, but once you go through all of her dialogue options, she’ll go to Majula. If you clear the Iron Keep, you can get unlimited Titanite Shards from her. If you clear Drangleic Castle, you can get unlimited Large Titanite Shards from her. If you clear the Undead Crypt, you can get unlimited Titanite Chunks from her. After the final boss battle, you can get infinite Titanite.
Cromwell the Pardoner – Enter the chapel at Brightstone Cove Tseldora, and climb the ladder near the door to find him. If your Faith is 35 or higher, you can get the Ring of Resistance from him.
Felkin the Outcast – He is in the Huntsman’s Copse. If your Intelligence and Faith are 20 or higher, you can get the Hex Set from him.
Lonesome Gavlan – You can find him at No Man’s Wharf, Harvest Valley, and eventually the Doors of Pharros. You can buy an unlimited amount of Poison Moss, Poison Arrows, Poison Throwing Knives, and Rotten Pine Resin from him.
Laddersmith Gilligan – He is initially at Earthen Peak, but when you either beat the boss from his section or buy his ladder, he’ll go to Majula. All the items he sells are unlimited.
Blacksmith Lenigrast – He is in Majula. Give him 8000 souls to get a Blacksmith’s Hammer.
Licia of Lindelt – She is initially in Heide’s Tower of Flame, but once you go through all of her dialogue options, she’ll go to the connecting area between the Tower and Majula. Once you get through the Shrine of Winter, you can get unlimited spells from her.
Maughlin the Armorer – He is in Majula. If you spend 1000 souls with him, he will give you Royal Soldier Armor and Elite Armor. If you spend 16000 souls, he’ll give you Alva’s armor. If you spend 16000 souls while you’re undead and have no souls with you, he’ll give you the Armor of Aurous.
He will give you armor for each of the following bosses you defeat: Smelter Demon, Looking Glass Knight, Lost Sinner, Throne Defender, and Throne Watcher. If you defeat all the bosses in a New Game +1 and +2, you can buy the Moon Butterfly Armor from him.
Steady Hand McDuff – He is in the Lost Bastille, behind a wall you can blow up near a barrel-rolling enemy. In a New Game +, he sells unlimited weapons and armor.
Merchant Hag Melentia – She is initially in the Forest of Fallen Giants, but once you go through all of her dialogue options, she’ll go to Majula. She has unlimited Life Gems. Clear the Lost Bastille to get limited Radiant Lifegems. If you spent 10000 souls with her and talk to her, you can get the Covetous Silver Servant Ring +1 from her.
She will give you armor for each of the following characters you kill: Warden Agdayne, Creighton, Benhart, Pate, Cale, Lucatiel, Targray, Straid of Olaphis - Black Armor, Licia of Lindelt - Saints Armor, Felkin the Hexer - Hexer Armor, and Navlaan - Chaos Armor.
Weaponsmith Ornifex – She is behind a locked door in the Shaded Ruins. Unpetrify the Lion statue by the Acid Cave to get the key. Once you talk to her, she will move to Brightstone Cove Tseldora. All of the items she sells are unlimited except for the Herbs and Green Blossoms, but the Green Blossoms will become unlimited after you defeat Nashandra.
She will give you weapons for each of the following bosses you defeat: Old Dragonslayer, Lost Sinner, Old King, Duke’s Dear Freja, Looking Glass Knight, Velstadt, Soul of the King, Guardian Dragon Soul, Ancient Dragon Soul, Throne Defender and Throne Watcher, and Old Witch, Old King, Old Dead One, and Old Paledrake.
Sweet Shalquoir – She is in Majula. If you clear Brightstone Cove Tseldora, you can get Flying Feline Boots from her. If you open the Shrine of Winter, you can get unlimited Prism Stones, Alluring Skulls, Lloyd’s Talismans, and Homeward Bones from her.
Straid of Olaphis – He is petrified in the Lost Bastille. Use a Fragrant Branch of Yore at the servants’ belfry bonfire to unpetrify him. If you clear the Undead Crypt, you can buy the Great Lightning Spear and Sunlight Blade from him.
He will give you weapons for each of the following bosses you defeat: The Last Giant, The Pursuer, DragonRider, Flexile Sentry, Executioner Chariot, Skeleton Lord, Covetous Demon, Mytha, the Baneful Queen, Smelter Demon, Demon of Song, Belfry Gargoyle, Ruin Sentinel, Scorpioness Najka, The Royal Rat Vanguard and the Authority, Velstadt, Giant Lord, Darklurker, and Old Witch, Old King, Old Dead One, and Old Paledrake.
Royal Sorcerer Vavlaan – He is in Aldia’s Keep. He sells unlimited Great Arrows. If you talk to him in Hollowed form, he will give you assassination quests. If you complete them, he will give you the Dispelling Ring, Simpleton’s Spice, Forbidden Sun, Unleash Magic, and his armor.
Vengarl of Forossa’s Head – In the Shaded Woods, go to the foggy area, keep to the left, and enter an area with stacked stones to find Vegarl’s head. If you find and defeat his body, he’ll sell Red Rust items in addition to his regular stock. Once you go through all of his dialogue options, he’ll give you his helmet.
Chancellor Wellager – He is in Drangleic Castle. He sells unlimited items. If you defeat the Giant Lord, he will give you the Royal Dirk and the Espada Ropera. His stock increases once after you defeat the Looking Glass Knight, and again if you defeat the Looking Glass Knight and the final boss during a New Game +3.
[more cheats for Dark Souls II here]