Just explore all your nearby forests until the icon pops up.

the seed: IronAge0
the trader is on the south east of your starter island and all bosses are quite close by

Yep
https://valheim-map.world/?seed=SXeUxXzILF&offset=474%2C164&zoom=0.175&view=0&ver=0.202.14
(Seed is SXeUxXxzILF)
Merchant is at walking distance to the east of spawn point. The closest merchant I've ever seen. Literally a minute running east

There's a guide showing the "best" map seeds; of which one has a trader at a walkable distance from the spawn.

The seed "wVJCZahxX8" has one trader at the southern part of the spawning island; while a 2nd location is at an island that's even possible to be reached with a raft and he's quite close to the coastline.

You can use the Valheim Map Generator to check out where to find the trader at the seed you're playing at too; if you don't want to restart your game.

You can also google the map generator. It can easily show you where everything is on the map. I wouldn't use it too much as an important part of Valheim is to explore and find stuff on your own, but it's useful for finding more rare things like the trader, or in my case, where the bees were after I had already explored my starter island. (turned out there were 2 out of 3 that I had missed)