Do the Mer trade in water or on land?
The Mer are just as happy on land as in water, they are not traditional mermaids in the sense that they only live in one way or the other. The mutable, changing element of water is inherent in their ability to shape shift. This gives them a yearning to be by the water, or in the water as often as possible.
There are a few land settlements along the coastline, like Thantis, where Mer enjoy the sunshine, forage for food to supplement their primarily sea-provided diet and explore. Given their attachment to the sea, they rarely wander inland preferring to stay within view of the sea or a lake or river. They enjoy the sunshine so much that many take small ‘holidays’ on the beaches on Pangaea, where they can have the best of both worlds; sun and sea.
Trading parties are often dealing in sea-goods that need to be kept fresh, so much of their trading with the other cultures of Pangaea is done on beaches anyway. This is part of the reason that Pyrrhos is such a central hub of trade, as it is in the middle of the other three cultures and surrounded on two sides by beaches, with easy routes between each realm.