What happens if I lose a scroll used in a deck?

Solution 1:

If you attempt to trade

When you click the plus to add the card to the trade. The Game Warns you that you are about remove the scroll and even tells you which deck it is used in. If you choose to proceed despite the warning the scroll is removed from the deck. If after that the deck is no longer legal (less than 50 cards) it will be moved to the bottom of the list of your 'Load Deck' list in deck builder. It will prepend the text [Illegal] in red to the name of the deck.

Screenshot:

If you attempt to sell

You get no warning, and the scroll is silently removed from any decks that it was in. If the decks are then not legal the same thing happens to them.

If you attempt to craft

You also get no warning. The scroll(s) get silently removed from the deck. And the newly "upgraded" scroll will get automatically added to the deck. However if you didn't have enough of the original scrolls the deck could still end up -1 card which can make it illegal.

Interestingly in all of these scenarios if the scroll get removed from the deck and it was the only scroll of its color in the deck, until you load and then save the deck it will retain that scrolls faction symbol on its label in the load deck screen, even though it no longer contains any scrolls of that faction. Despite this you cannot sacrifice for that resource in an actual game.