I've run into this issue too. You can still add it to your cart and checkout with it, but once it takes you out of the store and into the payment page, it says that there are inventory issues and the item is sold out. This seems to only be the case with the Digital Version; it doesn't stop you from buying the Physical Version. That said, it's confusing to see a digital item that's sold out. Hopefully someone can look into this once everyone has returned from Anime Expo.
Thanks for mentioning this. If you haven't yet, I'd really appreciate someone sending a support ticket in for this problem so i can make sure this issue is on our docket if it wasn't already. You can do that by emailing Support@Fakku.net.
We're going to have a lot of catch up to do now that AX is over, and this will ensure we address it as soon as possible.