The idea of having shops is pretty simple. Normally the player would have to build a marketplace to fulfil the citizens' need for certain resources. Something that would mean the player would have to build marketplaces all over their island even for residents at the end of the world. To avoid this, shops will be small service building working similar to market places just at a smaller radius.
Marketplaces are objects of prestige. Something to reflect the greatness of the player's settlement. They should not be just like any other service building.