On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the side quest Tiny Black Market.
Rewards: 1,200 XP.
At the location marked in the image above, you will come across a store in a treehouse. When you climb up, you will be stopped by a boy who wants to hear the password - answer him however you want.
After a while, a boy will invite you inside, where children are trading various treasures. Talk to the saleswoman, listen to the information about the items she sells and buy them all.