I don't know how recent this change is, but I thought I would share it for those of you who are interested.
If you're looking to buy something on Amazon Japan, check out the "available from other sellers" links (either new OR used). If the seller has it linked as Amazon Prime, then you can buy it! Normally you wouldn't be able to order the item, because the sellers ship only within Japan, and the checkout would stop you, but if it's marked as Amazon Prime, then you can (I think it must go to one of the local Amazon distribution centers, and thus it becomes available internationally).
You can potentially find really great deals on books and such. Just be sure to read the descriptions carefully about the state of the item. For example, you might want to stay away from 可 (passable) items.
Also, if you haven't ordered from Amazon Japan before, setting up an account is not difficult. Just be aware that certain items have restrictions (it'll tell you when you go to check out), and shipping goes via EMS, so to the US, each order has a minimum ~$30 shipping fee, plus $3-4 for *each* book.