In most jurisdictions, it is not "illegal" in terms of criminal law to sell your frequent flyer miles, but it is a severe violation of the airline's contract (Terms and Conditions). Airlines view miles as their own proprietary currency rather than the passenger's personal property. If an airline's fraud department detects that miles have been sold or bartered through a "broker" or third-party site, they have the right to immediately freeze the account, void all remaining miles, and even cancel existing tickets booked with those miles. In 2026, airlines use sophisticated "pattern recognition" software to flag accounts that suddenly book tickets for unrelated individuals in different geographic locations. While you won't go to jail for selling miles, the "civil" consequences—losing years of hard-earned loyalty status and being "blacklisted" from the airline—are significant enough that most travel experts strongly advise against it, recommending instead that you "gift" miles to family members through official, approved channels.