As a fountain pen lover, I agree with Jonas about the need to be extra-careful when buying them. This doesn't mean that all US or EU shops sell original products and other ones sell knockoffs, clearly. When buying a pen (and everything else) online, the first thing I check is the reputation of the store. That's why I would trust Goulet Pens, Cult Pens, Engeika, several Italian stores (as I'm Italian), and other known shops to carry only original products. There are also eBay users, like pisuke2005 or Engeika (they sell both directly and on eBay), known to sell original products and to have a good support service. By the way, the store Karl Blessing mentioned (the Bunkidou Shop, as Rakuten is just a Japanese online shops aggregator, pretty much like Amazon Marketplace or Etsy) might not be as famous as Engeika, but is known on several online forums for having original products.
In the end, I agree that fountain pens might require to be a little more careful when buying online, but that doesn't mean that all non-US stores sell knockoffs. ;)