The Castle Crashers Knight Prisoner is not intended to be marketable or tradeable.
It's a special Prisoner we created just for you, to thank you for owning both Castle Crashers and BattleBlock Theater on the same Steam account.
If you see the Knight up on the Marketplace, that is because the Prisoner was unintentionally tradeable/marketable when it was first released. If you purchase the Knight now, you will not be able to trade or sell it.
Thanks for understanding!