I would like to go to Seoul as a volunteer in a hostel and would like to know if the visa free entry as a tourist is enough for that purpose (not getting paid, just accommodation). Also, how exactly does the visa free entry work? Do I need anything other than the two plane ticket printouts?
I have lived in South Korea for few years and the Visa Free Entry would actually work for this purpose as you are not engaging in any paid employment. What is your expected duration of stay in Seoul?
South Korea Immigration is helpful and polite. They don't typically ask for any documentation if there is no Red Flag :-)
No. I wanted to work in a cafe for free accommodation, and was told that it was illegal. I was denied entry at the airport and deported. My experience suggests that legally it isn't.
Also, I would not recommend the experience of being deported. They throw you in a windowless room, keep bright white lights on 24/7 so you can't sleep, feed you nothing but sugar buns(2 a day) and tell you almost nothing. It's pure psychological torture and exactly like being in prison. I was kept for three days, and by the third day, it felt like my mind was unraveling, since I was so hungry, I'd had 4 hours of sleep max, and I hadn't seen the sun for days, and the officials were mean and supremely unhelpful, just telling me to go sit and wait all the time. It was a ghastly experience. Therefore just keep that in mind if you want to go work or volunteer in South Korea on a tourist visa, if you're caught, they treat you horribly, worse than a dog.
With regards to the "helpful and polite" comment above: perhaps if you're American. They treated me horribly, they asked for a lot of documentation, they were supremely unhelpful and they were cruel, feeding me almost nothing and keeping me constantly in the dark.