Originally posted by Zep Tepi:

If the search gets so long you don't want to play, go ahead and go to the map website https://valheim-map.world/

There, enter your seed number. You can select "no terrain" and only select "spawn" and "trader" to get an idea of what direction to go without spoiling the whole map. I did that after circling every island next to the start island and decided I was losing my interest in playing if I was just going to spend my next few play sessions wandering aimlessly in a boat.

That's what I did when I was about ready to enter the plains and still hadn't found him.

@OP: Keep in mind the first trader location on the map you explore is where he will be.