The bartering or selling of miles is against Aadvantage's policy. So if you do so, you'll be in violation of the t&c's of the program. But having said that, that hasn't stopped many people from bartering or selling miles. To answer your question, there's no 'reputable' way to do the transaction, since it's against their rules. I'm sure, though, that such activity takes place via Coupon Connection or sites like Craigslist.org