Well, I know I'm going to get flamed on this, but here goes ...
While I can't find anything in SWA's contract of carriage that prohibits people from selling unused travel refund credits, I'm sure it must be prohibited somewhere, and if it isn't, I definitely think that it should be.
Right now, SWA is about the only US domestic airline that lets you cancel or change any flight without paying a penalty charge (over and above any fare difference) to do it. With the recent changes to the RR program, it is easy to guess that if SWA is pushed too far on the Ebay front, change fees will be implemented. They are trying to be as kind as they can afford to be, because good customer service is good for the bottom line, but if a large market in resale flight credits develops, then I would not blame them one bit for wanting to get a share of it, and that will mean change fees.
Figure out some other way to get value from the credit ... please! Ask for an extension, donate it to a charity, give it as a birthday gift, but don't try to sell it on Ebay.