The black creepy-crawlies are from the Kannagi story; I've not watched the show but they seem to be representative of the impurity that Nagi opposes.
As for the sign that Zange is holding up, the translation could use a second look. Chuuko is simple enough (used, second hand) but shobun (dipose of, eliminate) could also be interpreted as a liquidation or clearance sale, especially in this context.
I think leaving it at 'second-hand disposal' is good, or changing it to 'second-hand clearance' if that makes better sense.
I figured out what it is. That's a reference to Nagi being "used goods" in the eyes of a bunch of mouthy otaku, and so Zange's using her as material for a "clearance sale".