Please don't take a chance selling your miles. It's not against the law, but it definitely is against the airlines' rules and regulations. It's fairly easy for them to spot an illicit transaction and you just don't want to chance it. Give the miles to your relatives for their exclusive use or give them to charity and you may be able to get a tax deduction. I've had considerable experience and please believe me it's just not worth even considering it. United and AA are particularly adept at determining these types of transactions and will penalize you heavily if discovered. They also have the option of canceling your account(s).