One reason I can think of is that the posts on this forum are indexed and return on a simply Google search. Do you really want the world at large and people who know your "handle" to know each and everything you're buying? It would be a LOT different if the site wasn't scraped by BigData.
Just me, but I think if you try and sell small amounts on the forum, you'll build up a reputation as being reliable. I (personally) wouldn't sell to a refiner as they'll a) pay you way less than spot (they need to make a profit), and b) technically it's illegal to melt down Australian currency...