Only fully researched buses can be purchased. Research can be conducted inside the Design Firm. When the bus has been fully designed within the Automotive Plant building, you will be able to purchase it, and send it to the Automotive Plant.

Buses cannot be upgraded, but you can research and design new types of buses.To sell a bus, navigate to the Bus Station building, select the bus you’d like to sell, and press on the trashcan icon. A confirmation window will pop up, showing you how many coins you will receive for the transaction. Once you confirm, the bus will disappear, and the coins will be added to your account. Buses cannot be sold if they are currently out transporting passengers, or if you haven’t grabbed the income from the last trip it made. Wait for the bus to complete its route, grab your money, and then you are free to sell it.
 
Have fun playing the game!