Is it permissible to sell a Quest company subscription after realizing that dealing with it is impermissible, and is expiation (kaffarah) obligatory for me?

If the buyer will use the site for an unlawful purpose, then selling it is impermissible, for the Prophet, peace and blessings be upon him, said: "Indeed, when Allah forbids something for a people, He also forbids its price for them." This is also because it falls under aiding in sin and aggression. As for what you previously earned from unlawful work, there is no blame upon you for benefiting from it, and you are not obligated to donate it, because you were unaware of its prohibition at that time. There is no expiation due upon you, and you must not return to such actions in the future.