There already is one.
lol. I would rather stay off JSP as much as possible.
To the top!
What do you think would happen to a trading forum here? It's the same people with all the money, whether they hang out here or on jsp. The reason I started going over there is because that's where the money and best items are. Also, if there were a HC trading forum officially sanctioned here, then Blizz wouldn't get their 15% cut of HC gold (which is actually worth something, unlike SC).
edit: just went to check it out. JSP's site is much better and has a larger audience. No point in splitting the trading up imho.