I've seen a number of sites advertise that they'll buy your miles. They seem fairly schlocky, so I was wondering if (a) they were real, and (b) what the business model is...

•.http://getpeyd.com/
•.https://flipmymiles.com/
•.http://www.miles4money.com/

Are they just the back-end to travel agents, who are using the miles to buy business and first-class tickets for less than the listed price, and then pocketing the difference?