Rakuten began recently to take the international shipping out of the hands of individual sellers and started handling it themselves, just like Ebay. It got easier to order through them, but the individual sellers can presumably still opt out from that. From the buyer's side you can use the
Rakuten Global Express, establishing a personal address in Japan - no own experience with this.
I actually signed up for it but received an email when I bid on a Birdy and was told to cancel my bid because they would not be able to ship the bicycle. When I pointed out that I was going to use Rakuten Global Express, they stated that it would not be possible due to some restrictions about bikes. So I gave up.