Core Keeper NPC houses need to be built properly if you want the friendly little merchants to live at your base. As you explore the underworld of Core Keeper you'll find powerful boss monsters, and when you defeat them you'll gain access to special items that can be used to summon wandering merchants to come live with you...
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Core Keeper: How to get NPC merchants to live in your base
Source: PC Gamer (opens in a new tab)