When you want to make some money, you can sell your tiles on the market for a specific price and wait for a buyer. Open the drop down menu belonging to your account name at the top right of the page. Open the Insights page.
From the insights page, you can click on the sell button behind the land that you plan to sell. This will open the following dialog:
You can type the price per tile you want to sell for, and click on advertise to confirm. Now you tiles are available on the marketplace, waiting for a buyer!