This interface should be implemented by a marketable entity (CatalogItem). This interface should be implemented by a marketable entity (CatalogItem). This interface should be implemented by an entity that can be offered on a market (Offer). This interface should be implemented by an entity that can list Catalogable elements (Repository). This interface should be implemented by an entity that can be sold for a price like Offer. This interface should be implemented by an entity that can order things.