There are normally three spawns withing the map world. The one you find first will spawn the trader and the other two will vanish at that point

Also, if you don't mind peeking, there's a spawn generator online that'll show you where everything on the map is.

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

try this out, just note your seed from your game first.
https://valheim-map.world/