Map generation is based around the initial spawn point on the map. If you drew concentric circles from the initial spawn point, you would find all the safe places to be, then not so safe, etc, etc. Therefore if you search the initial circle (inside a day of sailing), you'll likely find the trader... he's got about ten locations he 'can' spawn in, until you find one and then he's locked there.

My first search pattern was just random dark forests.... That was a ♥♥♥♥♥♥ plan.