For problem number 1, I suggest putting the Wanted To Buy section between the For Sale section and the SAU Traders on the homepage. This way people would see the Wanted To Buy as it is just underneath.
For problem number 2, I think that allowing the traders to post throughout the forum is a good idea, as long as they are not trying to excessively flog their stock. I believe that the knowledge of some of the traders could be very useful throughout the forum.