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.