Board Thread:Questions and Answers/@comment-173.76.105.25-20160304024208/@comment-26447088-20160901181003

I believe that happens if they're in your Raid or Quest deck.

Steps to find the page:

Menu>Edit Deck>Raid Battle Deck

If you have the unit in any of Deck 1 through 5 (Auto doesn't count for the Deck Unit status), you can't auction it off. Units in Quests decks also can't be auctioned as well, I believe (excluding Auto). Deleting the deck will clear all units in it, also removing the Deck status. Otherwise, you can replace it with another unit.