I'll stay at Shanghai Marriot Hotel City, where is the closest place to buy sightseeing bus ticket?

You are advised to take it nearby the People's Square Subway Station. The exact location is No.100, People's Avenue. From your hotel, just walk southeast around 0.6 mile to find the pick-up station. After arriving, you can easily find a ticket booth near the bus stop and purchase tickets there. Or you can just pay cash to the bus driver onboard.