To buy a new house, you wanna go to the Game tab, then Housing tab, and you should see "Buy New House". That's for buying a house directly from the server, and then there's the player market "House Market" but currently nobody is selling their house.

To sell your house, its status needs to say Uninhabited. You can check its status under "My Houses" (Housing tab again). The thing is, you can only change its status to Uninhabited by moving into another house. Edit: forgot to say, once it's uninhabited, you click House Market, and your uninhabited houses will be listed so you can put them on the market.