Filter already purchased items in the store

I would like to have the ability to filter already purchased items in the store. With hundreds of doujinshi and books, it can be easy to forget exactly what is in your library or not. This makes browsing the store and filling your cart tedious and has stopped me from making purchases on multiple occasions.

I would have assumed there'd be a tick or something on it, or 'owned' in the add to cart box or on the thumbnail or something already, so you know