I am planning a flight to Bangkok arrival in Seoul 0635 AM and departure at 8:40PM. I want to go to Itawon shopping during this layover and need some transportation advice from the airport. Thank you.

You certainly have enough time but the city is a long way away from the airport. I would budget at least an hour each way.

If you do a search (using the text search box at the top of page), you will likely find another thread about this. There are at least two good airport bus services that will meet your needs; the KAL one is more expensive, and has plusher seats, but both were comfortable touring style coaches. Just find the ground transportation desk at the airport and they can direct you. Traffic can get pretty snarled in Seoul, so allow plenty of time for the return.

To depart the airport in Seoul might be an expensive proposition - will you have to pay departure tax leaving for Bangkok because you left the airport???
There are two transit hotels one at each end of the terminal. $52US for six hours, bed and shower. Free internet service is available also at both ends.

I currently live in Seoul in the Hannam-dong area near Itaewon. One of the airport buses stops in the heart of Itaewon, just ask at the information desk. The fare is 12,000Won each way. If you are travelling on a Friday give yourself at least 1 1/2 to 2 hours to get BACK to the airport. Traffic is amazingly bad on Fridays. If you are planning this layover during the Lunar New Year holiday (Jan 21-23) then most of the Itaewon shops will be closed, so you may be disappointed with the shopping. Also remember to check on visa requirements--Americans need a visa to enter the country. Good luck!