Two types of Star Prisoners get released from time to time:
- Feature Prisoners: These get unlocked when you finish a specific Furbottom's Feature! They are generally available for about 1-2 weeks, though this can change.
- Free Unlocks: These get unlocked when you log into the game! They generally coincide with a holiday or special event.
In both cases, the Star Prisoner will NOT be marketable (can't be traded or sold on the marketplace) while he or she is available to everyone.
However, each Prisoner becomes marketable after he or she stops being available as a general unlock. Once a new Prisoner is released, you can trade away the previous Prisoner to your heart's content.