There was potentially a lot of profit as Bitcoin traded for 10-20% more in S Korea and the liquidity in the market was in the billions. The tricky bit would be once you've sold your Bitcoins for Korean Won, how easy would it have been to change the Won back to USD given S Korea had capital controls at the time. I'm not sure how many you could get away with changing by going there as a tourist. Probably a limited amount. Not sure if anyone knows how much?
My understanding of this trade is that you can trade unlimited amounts of Won for USD as a person outside of Korea (beyond the jurisdiction of the Korean government), and as a non-Korean you can bring out an unlimited number of Won as long as you don't carry it as cash. However, this is all a set of "loopholes" that aren't exactly illegal but aren't explicitly legal, which is why no reputable bank or trading firm could do this trade.
The risk that you end up getting banned from entering Korea for doing this trade too much was real, but I don't think anyone who tried this trade faced any of the possible consequences.
I love a good arbitrage technique, but this one really just reads like an excuse to go to South Korea with your friends (or your polycule).