According to my online research, in order to travel from Kalambaka to Ioannina by bus I first have to go to Trikala, then take another bus to Ioannina. This same bus seems to travel back through Kalambaka en route to Ioannina. Is it possible to simply purchase the ticket in Kalambaka and board there?

The Ioannina bound bus leaves from Kalambaka 30 min later than from Trikkala from/to IOANNINA :: ???? ???????? ??

So I can purchase the ticket in Kalambaka and board there for Ioannina--no need to go to Trikala?

Of course! But may be not at the last possible moment.
Most bus tickets can be bought in advance.

You can of course buy the ticket in Kalambaka . the bus is coming from Trikala, stops in Kalambaka in order to pick up passengers and continues to Ioannina.
The trip Kalambaka- Ioannina takes about 2 hours and you can even buy tickets online from the KTEL Trikala website ticket prices :: ???? ???????? ??

Settled!
Thanks, both of you, for your helpful posts (clausar, neckervd)!