From this page of the Valheim game guide you will learn how to locate Haldor the Merchant.
Where can I find the merchant?
Haldor is a merchant who can always be found in the same, predetermined location on the map. However, this location changes based on your map generation seed. Because of that, it's impossible to pinpoint exactly where he can be found during a particular game. You can only somewhat narrow down his possible location. He will always be stationed in the Black Forest area. Consequently, you might need to embark on other islands to find him.
What does Haldor have in stock?
Some of the items in the merchant's inventory are impossible to find while exploring the world. These objects are listed in the table below.
Illuminates the area in front of your character
A belt that allows you to increase your character's carry weight limit by 150
Crafting material required to create high tier equipment
Allows you to catch fish (requires fishing bait)
Sold in stacks of 50. It is a prerequisite for fishing.