Being relatively new myself, I think I've been truly active on the boards for about 3-4 months now, although I signed on almost a year back. I started off buying and dealing with the sellers before I put up a few things for sale. There should be some qualification to become a seller, though. As you have seen on this thread and others, sometimes the terms and conditions of the sale are not clearly expressed in the sales thread.
I like the way that the forums are self policed - maybe I should have screwed up on my initial sales attempts just to get the publicity.