Shadow Listing is a list of people mainly in the Art community/Adoptable Community who are on a thin line for getting something from an artist.
A way to get Shadow Listed is by complaining about unfair rules in a Artist's/Creator's post when you do not get something, whining about how a Artist/Creator decides to keep a Adoptable, or you have harassed someone the Artist/Creator knew/is friends with.
All it does is lower the possibility of the person getting accepted, even if they are the only one who offers/enters.
I'm Shadow Listing Marble, so now she doesn't have a high chance of getting the Dragon Adopt.