I like to say: It is not that easy to find Haldor in Valheim. Every map is autogenerated using seeds. That means that every map is different. Sometimes it is easy to find him, sometimes its not. 

This guide could help you to find him “more” quickly:

• Haldor will spawn in Dark Forest biome. 
• He will not spawn in a 2km radius arround the starting point
• He will spawn close to the starting point but not in a 2km radius arround the starting point. 
• He mostly will not spawn on the starting island. 
• You need a ship to find him.
• If you did not find him yet you will see a “backpack icon” on the map if you are in a 2km close range to Haldor. 

I've found him on different maps. It was the best way to get on a ship and sail around the island. By doing this I was able to find him every time. I realy enjoy finding him because it feels like a little “adventure” to me.

Using this seed will help you to find the Haldor the Merchant:

You could also try this seed: 42069lolxd which was shared by some other player with me. The merchant is south-east from the spawn point 3-4km. Keep in mind: You could start a new map with this seed, searching the merchant. You could take your gold with you from your main server, buy some stuff and go back to.