A friend wants to make a plane trip using miles from her Capitol One credit card. She doesn't have quite enough miles for the trip. Unfortunately, unlike airline-specific miles, Capitol One doesn't let you buy extra miles, or pay with a combination of miles and cash. However, they do allow you to transfer miles from one person to another. So if anyone has a Capitol One card and would be interested in selling some miles, I have a buyer for you.
You'd think that someone would have set up a web site where people who want to buy miles can find people who want to sell miles, but I haven't been able to find one; I can find "we want to buy your miles!" sites, but no "you can buy miles here" site.