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Back in 2017 if I remember, the premium was 10%-20% for month after month, with billions being traded. Some people must have made a lot of money by knowing how to convert large sums of KWR back to BTC (presumably via USD).

My guess was by exporting electronics or something similar. USD -> US exchange -> BTC -> KR exchange -> KRW -> electronics -> ship to USA -> USD. Of course a very friendly banker might provide an easier route.

Non-Koreans can't trade on Korean crypto exchanges any more (but maybe can own a Korean company that does?). Iran and Venezuela seem to have a premium for obvious reasons, but again it's hard to trade the loop.
 
Back in 2017 if I remember, the premium was 10%-20% for month after month, with billions being traded. Some people must have made a lot of money by knowing how to convert large sums of KWR back to BTC (presumably via USD).

My guess was by exporting electronics or something similar. USD -> US exchange -> BTC -> KR exchange -> KRW -> electronics -> ship to USA -> USD. Of course a very friendly banker might provide an easier route.

Non-Koreans can't trade on Korean crypto exchanges any more (but maybe can own a Korean company that does?). Iran and Venezuela seem to have a premium for obvious reasons, but again it's hard to trade the loop.
Companies can trade cryptocurrency but cannot deposit or withdraw KRW.

Foreigners with proper VISA can trade.
But I don't know if it is hard for foreigners to get bank account in Korea.
 
Companies can trade cryptocurrency but cannot deposit or withdraw KRW.
That is an interesting approach. Allow crypto in principle but isolate it from the official currency.

I don't know if it is hard for foreigners to get bank account in Korea.

They can with a residence card and proof of employment. I don't think it's easy to convert large amounts of KRW to USD at the published exchange rates. Hence my guess that you would buy some other commodity to export.

I've had luck when some specific crypto is more popular in one country than another. e.g. OMG had quite a premium over BTC in SE Asia for a while. Then XRP. It's much easier to trade something like BTC -> US Exchange -> OMG -> Asian Exchange -> BTC than involve fiat currency, but the spreads and volumes tend to be smaller.