I have a moral question to ask? Why is buying and selling a bad thing?
To me, buying and selling is a service to both sides. However, like NYnoob said, a fraud(which is in his original post) does no service to the community. His content seems to be more about fraud and not contributing to the community.
Getting FREE info from the forum is a plus to the community and getting quality products from a respected member or vendor is also a plus to the community which enrich our collection of the "precious" and more equipped to deal with the horror or reefing nightmare. So the question should be addressed back to fraud not people who just come here to sell or buy. THEY DID NOTHING WRONG. A person who come on the site just to sell does not necessarily be a bad person at all. Do not confuse with cheaters. Cheaters can be within our community for a long time and may have lots of posts. That's where the traders rating comes into function(sometimes not very functional though due to the user's fear of reprisal.) Do these sellers use up the "FREE information grapper's" resources? YES. So, for practical reasons(such as needs of finance to run the servers) and not for moral ground, I would opt for only donors and/or vendors can sell.
Note: One does not have to go to the Market Place forum either. One can stay in the forums of beginners, advanced, nano, ..... there are too many to list. Why controlling someone else's freedom of selling as long as they are not doing harm to us?