This page of the guide will let you learn about trading in Horizon Forbidden West and how to unlock traders.
Horizon Forbidden West's first traders can be found right after the prologue, once you reach Chainscrape. Unfortunately the traders won't trade with you at first as they consider Aloy a stranger.
Traders - how to unlock?
Traders in Horizon Forbidden West are fully unlocked once you progress with the main storyline, specifically after completing the To the Brink quest we described in more detail on a separate page of this guide.
After Aloy solves the problem involving Ulvan, Chainscrape will reopen for trade and you'll be able to trade with any encountered merchant.