When you've got 90 posts on FT you can access Coupon Connection. This forum will allow you to effectively 'swap' your miles for hotel points or something else you can use. In effect, you would use your miles to book a flight for someone else, whilst they would (for eg) use their Hilton points to book you a couple of nights somewhere. Everyone wins, but because it requires a lot of mutual trust there is this restriction on getting access to the forum.

Trying to sell miles privately is a bit of a minefield, especially given BA's availability of reward seats. You can't really go on e-bay and offer to book someone a J seat to San Francisco, only to find that there are no seats in the month they want to go, and of course you need to get the money off them for the taxes. And if they had to cancel, the miles would go back to you and not to them.