Sadly it looks as if you can't see what they have for sale unless you spoof a Japanese IP through a proxy. Too bad, I hoped this might provide me with a way to buy Japanese ebooks from the US.
From what people are saying on Japanese boards, there are only about six thousand or so paid titles (the rest are public domain, for a total of around 18 thousand).
http://wildhawkfield.blogspot.jp/2012/07/kobo_19.html <--This page provides some info about the store (in Japanese). If you click on the Google Docs spreadsheet link in the blog entry, it shows the distribution by genre of the works. Most are novels.