I have a client who wants a little online store, it will have relatively few items, no categorization (maybe something very basic, maybe tags). Expected market is all over the world (US, Canada, UK, Israel, Russia, Ukraine, Japan). So the store needs to be able to accept as much payment methods as possible. (Paypal is not widespread in Russia, for example, due to certain limitations with its service.) Accepting credit cards (visas, master cards, etc.) is an absolute must.
While I am a front-end developer, I don't fear back-end programming and I've experience with Django, Rails, Sinatra and all kinds of PHP (though I would prefer to not work with PHP again). I have also worked with liquid templates so theming Big Cartel or Shoppify is not a problem too.
What are the things I need to consider if I will decide to build the store myself? What are the problems I will face when implementing different paying methods? What are typical problems with online stores?