As you progress through Death's Gambit, you will find many different items, but the problem is that you won't find NPCs willing to buy them. Even when you find yourself in the Sanctuary, there will be many NPCs selling various items, but none of them will buy anything from you.
Thus, you need to find the Sorcerer - this is an NPC who can improve your items and make them more powerful and effective against high-level enemies.
To find him, leave the Sanctuary and head to the upper left, past the shopkeeper. After that, kill the spearman and two shooters and continue to move on. In the end, you will find yourself in front of the boss Phoenix - kill him and then head left into Obsidian Valley. In the valley, head left again where you will find two NPCs.
From here, go straight until you get to the horn - interact with it, and it will lower the stairs at the beginning of the Obsidian Valley location. Climb the stairs and you will come to the Tundra Lord boss.
Defeat the boss and go left, where you need to rescue the bound NPC - this is the Wizard. Once you save him, he will appear at the Sanctuary. You will be able to find him next to the idol of death.
The sorcerer can enchant your items, give you soul stones, and can also upgrade your weapons to make them more powerful.
