So where is it possible to buy the ticket for the tourist bus from Wangjingxi to Jinshanling?

I am afraid it's impossible to buy the ticket in advance. When you get on the bus, the conductor walk to you and ask you to buy the ticket. If you have Beijing Smart Transportation Card, you may be asked to swipe the card before you get on the bus. Generally, there will be more tourists at weekends than working days. So it's very likely that there will be over 20 tourists.

Okay, thanks! Today some person from a tourist information office told us theres no direct bus so I was confused, but I guess she just tried to sell their own tour. So basically we just have to be at the bus stop early enough to queue to get on the bus, not to buy tickets since we have the cards