Is it haram to sell an airline ticket unofficially to make money, knowing that officially canceling it would have resulted in zero refund of its value?

The general rule is the obligation to fulfill all conditions in contracts, as long as they do not violate Islamic law. Permissible conditions agreed upon by the contracting parties must be adhered to. So, if your employer stipulates that you must not work outside official working hours, you must abide by that. And if an airline company prohibits a passenger from selling their ticket to someone else and stipulates that the benefit must be utilized by the passenger themselves, then this must be adhered to. However, if no such condition is stipulated, there is no objection to selling the ticket.