I'm going to be picking up my mom on Monday at the Incheon airport. My original plan was to buy a ticket at the bus station in Pyeongtaek but now I have a doctor's appointment that morning and we will be close to the Seoul Express Bus Terminal. Can I get a bus ticket that leaves from there and goes to the airport?

There is a Downtown "Airport Bus Terminal" around there -let me check.
My old Lonely Planet book tells of many buses from the Express Bus Terminal to the Airport. Schedule is old. There is an Airport Limousine Bus (Big) from most large hotels every 30 minutes. It was 18000 per person from the Hamilton when I went in March. I came back downstairs on a local for 7000, thanks to the girl at the info counter.

My cousin went from the Airport direct to Songtan bus station- think she paid 12000??? You do not need to go to Seoul. In fact, it is out of your way.
Get a trusted Korean to call Songtan bus station.

Once I pick my mom up I plan a catching a bus back to Pyeongteak. I know they have a bus station. I'm going to be in the Yeoksam area earlier in the day that I have to pick her up and I saw that the Express Bus Terminal is close to that area. I was wondering if I can get a bus from there to Incheon.

Hi My friend !
Check it out at below link.

http://www.airportlimousine.co.kr/route/route_eng.htm

And you can take a subway at Kimpo ariport subway station.
You can go to Incheon International Airport from Kimp Airport subway station.

Thanks to SHSONG for the bus map! Just click the number of the bus and see where it goes to the Airport!

Guess I recommend talking to the Pyongtaek Bus Station ahead of time for times and prices. Not speaking Korean can be a problem at the Airport, so know that local buses are on the lower level, and Express Buses are on the higher level. Get the Departure location straight at the Information desk upper level, or find the USO Desk and verify before dragging bags all over the place! Good Luck. Don't change money at the Airport, the rate is low.

P.S. For others who may prefer the subway- great ride without bags! No way would I drag 100 lbs of bags or more from The Airport to anywhere on escalators and stairways, particularly if there was a line change on the route!
The bus is made for that. So it costs a little more! If you are rich, and like to be scared to death, take a taxi.

I know USO Desk. That is the end of 1F passenger terminal at landside.
They are service for U.S. Army but you will give can some help to them.
My friend ! find it. they will give you some good information.

Shsong, it looks like I need to use bus 600 and that it leaves from Express Bus Terminal 3. The only thing that is confusing me is that according to the map on the website it says that Express Buss Terminal is on line 2 but according to my subway map it is on line 7. Has anyone been to the Express Bus Terminal? I'm wondering if it is easy to navigate around the place there?

Oh..my friend ! Express bus terminal is line 7.
If you take a subway at Pyeongtack subway station,
First, you have to transfer to line number 4 at Kum-jung subway station for Dang-go-ge and then, you have to transfer to line number 7 at E-su subway station for Jang-am. So, you have to get off at Express bus terminal.

If you have any question, please tell me or you can tell to FBI Agent Mulder and Scury.