I've been in Seoul for almost 3 Months this year. From what I've seen there are quite many rooms being rented on Craigslist and many of the posts are serious and not some rip-offs like in other places. I've stayed in a room from Craigslist and it was great and I saw there were quite a few others too.
Personally I would not recommend Itaewon because it's really a foreigner ghetto there. I've been in Songpa (one of the less central areas) and I was quite happy there. Obviously many people do not speak English, but I've had the impression they usually try their best to be able to communicate, and learning the Korean characters is super easy which also helps (took me about 4 hours).
One of the things I really enjoyed in Seoul is that there are tons of 24h cafes, which was great for me because I usually work at night.