You are a real estate agent who has a certain amount of cash, and has to buy and sell buildings to make a profit.
Your ultimate goal is to make as much money as possible in the shortest time frame.
Basically you have to be clever and buy the buildings when they are cheap, and sell them when they reach their price peaks.
When there are new building appear, if you have enough money just click on the building to buy it. If you want to sell the building you have, simply click on it as well.