I thought it would be cool if I built my market on a floating raft in between islands, so I built north from one of my islands, and placed the market on a raft 5x5 made of bridges. I then later remembered that you can buy land. I bought the island where I built my market, and it didn't destroy my market, and it's still functional, but I can move through my market as though there's nothing there. I'm not sure if monsters are implemented yet, but this seems like a potentially abuseable bug if I can walk through something, but NPCs cannot.