Is selling goods that contain forbidden or questionable ingredients considered a sin?

It is not permissible to sell forbidden things, because if Allah forbids something, He forbids its price. As for that which is doubtful, it is better to avoid it to clear oneself of responsibility, for "the lawful is clear and the unlawful is clear, and between them are matters that are doubtful, which many people do not know. So whoever guards against the doubtful matters, he has indeed cleared himself (in regard) to his religion and his honor. And whoever falls into the doubtful matters, he falls into the unlawful."