If you buy drugs off someone on a public forum you deserve everything you get for being so stupid. It is either a fed honeypot or some other kind of scam.
If some people from the underground source forums start public sites there could be public forums that sell drugs with no feds selling. It is all a matter of screening, not publicity. If one hundred screened people publicly sell drugs on a forum that anyone can access then there is not much risk. A public and free-for-all forum would be a horrible idea though.