The buying and selling is fast and easy to automate. The difficult part is in transferring non-bitcoin currency to and from the exchanges. If you for example

The buying and selling is fast and easy to automate. The difficult part is in transferring non-bitcoin currency to and from the exchanges. If you for example buy bitcoin in the US and sell in China, then you end up with a surplus of CNY on a Chinese exchange's account. Getting this converted and transferred back to the US will incur exchange and transfer fees. The transfers might also take weeks - which will limit you to cycling small amounts of money slowly, or risking a lot of money at once if the exchange "has banking issues". In the end, even a 5% or 10% price difference might not be worth running the arbitrage cycle on difficult to deal with exchanges in strange countries.