Right now, selling is forbidden, there are some general rules herehttp://www.canonrumors.com/forum/index.php?topic=1442.0
Craig is in the process of moving to totally new forum software, the new software is more friendly towards setting up a buy and sell forum, but don't expect it to happen soon, just getting the new forum up and running will be a big job.
It is on his list of things to consider. The administrative work and moderation effort might require a fee to be charged, and, of course, the legal implications of having buyers or sellers get scammed are a concern.
Fred Miranda has a buy and sell forum, and charges a token fee so check it out.
However, if you are a software publisher or writer wanting to sell your software, its best to use ebay, any forum will almost certainnly be for sales of personally owned items, not for businesses.