Many local laws differ from country to country, because of this its important to research before entering a new market for the first time. For example, trading within the EU is likely to be much easier, due to the common market legislation that’s already in place, but even so it’s sensible to check before you take on any European clients.
Remember that overseas clients will pay you in their local currency. This can be the Euro, dollar or any other option. So have a clear understanding of the payment.