We allow people to sell these now and then, under the assumption they have some extra rewards they'd like to get a few dollars for. However, if you want to sell them on a regular basis, it becomes a commercial enterprise, and you need to pay for a Classified Listing or Commercial Marketplace ad.
http://forumserver.twoplustwo.com/19...fied-listings/
http://forumserver.twoplustwo.com/17...l-marketplace/