You may sell your airline miles through any airline miles buying company. To sell the miles you need provide your contact details and need to tell how many miles you would like to sell. And you need to notify about the airline name and the expiry date, finally asked to provide the asking price for those miles. After the transaction is complete you will receive payment .Visit here https://sellmyrewards.com/why-sell-your-points/ to know about the complete procedures of selling airlines miles.