wow, that's crazy
apart from credit card payment, are they monitoring PayPal or other online payment methods?
Not sure about paypal but as long as paypal is working based on credit card or withdrawing money from bank while you are abroad, I think there might be a possiblity of getting caught.
Just for info paypal f&f , wise is all banned in south korea too.
So the only thats is feasible around this country is
1. send the money to someone in japanese who you can trust in multiple split in few months
2. that person buys the watch for you.
3. you come in with your watch o your wrist.
4. get the watch sent via other method.
5. Even with all these, if you get caught when you enter korea with watch on your wrist. uwill have to provide the evidence, that u have bought the watch within korea.