The Dream Ball cannot be legitimately bought or sold.
In the Entree Forest, when a player "speaks" to a Pokémon, a Dream Ball appears in the player's Bag. As Poké Balls never fail in the Entree Forest, and the Dream Ball can only be used there, the Dream Ball effectively never fails (having a catch rate modifier of 255×). However, any Poké Ball in the Bag can be used in the Entree Forest, not just the Dream Ball. Pokémon transferred from Pokémon Dream Radar are obtained in Dream Balls.