This office is now closed, so you have to book on line, rather than in person for all City link departures.
You can still buy on coach if there is room.
My plan C in case of poor weather on the Clyde this morning was Coach to Oban for Craignure, booking on arrival at Glasgow at 0645.
Luckily that plan wasn't needed, as I don't do credit card transactions on public wifi.
Well that is a drag that they have closed the office.
Glad you didn't need it today at least.
This is an update. I have just been to Buchanan Bus Station. A new temporary office has now opened at the very end of the concourse, past the public toilets, at the junction of the stances 1 to 42 which are for local bus services.
In Glasgow you can also buy Citylink Coach tickets at the Visit Scotland TIC.
Note that there is no access to the public toilets for departures before 7am and after 11pm.