In most jurisdictions, including the US and UK, it is not illegal (i.e., not a criminal act) to sell your airline miles. However, it is a major violation of the Terms and Conditions of almost every airline loyalty program. Airlines view miles as their own property, not yours, and they strictly prohibit the "bartering or sale" of miles for cash. If an airline detects that you have sold your miles to a "mileage broker," they have the right to audit your account, cancel the issued tickets, and permanently ban you from the program, causing you to lose all accumulated points. While a large industry of mileage brokers exists, using them carries a significant risk. For most people, the safest way to "sell" miles is to use them to book a flight for a friend or family member, as long as no money is exchanged in a way that the airline can track through a public marketplace or broker.