You start with 10 possible locations for the trader. The one you find first becomes it.
Out of those ten I would guess it is probably split between right on the shore. Inland some, or only find by going inland on foot. I have encountered all of these situations in different worlds.. I have even had worlds I never found the trader in... probably due to many of the possible locations being far enough inland to not find by sailing. Not like I checked every inch of the world though.