Nothing. Consignment shops don't own the items they sell. They only serve as a holder of the item. You retain ownership until a customer buys the item. The consignment shop usually works on commission. They don't pay you, you pay them.
The price that a consignment shop should pay on barely used outfits is negotiable, meaning it's not fixed. This is due to the fact that the price would be dictated by how much the buyer is willing to buy and how much the seller will accept.