Is there any place at Beijing Capital International Airport to buy tickets for domestic flights?

Yes, you can purchase the flight tickets at the airport in those three terminal buildings. The exact locations are the following:
T1: Departure Hall Central Roundabout and Counter No.K01 of on Floor 2
T2: Ticket Counter of Departure Hall on F2
T3: Ticket Counter of Departure Hall on Floor 4
The airport has many clear English signs that can help you find the right places. So don't worry.