unfortunately its all random where you can find the trader. often they are on a different island. If you keep just wandering eventually you will see a money bag symbol pop up in your map that is a spawn location for the merchant.

If you dont want to just randomly wander you can, if you choose to, you can go to valheim-map.world and put in your world seed to see possible locations for the trader. I did that when I was getting no where finding him. I really wanted to strength belt :P

Hope that helps at all