When you go to your nation's 'Trading Card' page, could the list of your 'Collections' on which you can click also be given an "Uncollected Cards" entry that you can click on to see a list of the cards that you own but that you have not placed in any collection? This would make it easier when deciding which cards to junk if you need to make space in your deck or to raise a small amount of Bank for some purpose.
(Idea inspired by imgur's 'Non-Album Images' option...)