The seaposts are very minimal on the amount of wares they sell, and it is odd that they don't sell fishing related wares. Particularly considering that they are supposed to be fishing focused seaposts.
One way to fix that is that, among the random items they sell, they could sell fishing poles at the sea post. They could always sell at least one fishing pole. That pole could cycle in type similar to how the other random items they sell cycle.
It could also be nice if the post sold some sort of bait. Either for the area they are in, or bait for another region which would require the buyer to travel to get to the region that they want to use the bait in.
