Don’t forget to use escrow to protect yourself and anyone buying coins for you in the US. Escrow means if, for example, you want someone to send you, say, 3 BTC for you to sell in South Korea, then you must first deposit 3 BTC (or more) with a trusted escrow listed here:
https://bitcointalk.org/index.php?topic=855778.0The purpose of this is to protect your source of coins in the US from being scammed.
This means for you to become an arbitrage broker you should have some kind of collateral or capital to be escrowed. This obviously makes sense because this is a transaction between strangers.
Incremental amounts are just as risky and will require the same arrangement described above.