This is currently possible, but I believe it may be a part of the premium package - and you need to have settings set a certain way.
1. Have all your decks marked as Built:
2. Set how you want Deckbox to look at sharing cards between decks:
3. Then in inventory you can sort by How Many Needed in All Decks (Increasing)
I think this is basically what you're looking for.
In theory from here, you could select them page by page and apply a Tag.
Then take the Tagged result page and export that as a CSV. At this point, if the goal was to set them all for trade, you could delete all of the cards you just exported from your inventory. Go into the spreadsheet and change the trade number to equal the inventory number then reupload the cards to inventory - now all those cards are tagged for trade.
If you're a premium member, and you want to trade anything you aren't using you can also just click here:
All that said, a feature in Inventory to see In Deck/ Not in Deck would be great! I would definitely use that filter.