A whole forum. We've already gotten several threads in the general forum about trading, and I think a lot more would be made in a Trade Forum. Right now, shouting on the actual forum asking for a trade is the main way of trading. It's incredibly inefficient. With a trade forum, trades could be organized in advance and even take place while both parties are not online.
For example, one could agree upon a trade, make a chest, chestallow the other trader, and put in what they are trading. Based on trust, the other trader takes what is in the chest and deposits the agreed-upon payment. That's really no different than the current trade system; some inherent risk. Violators of this code will, of course, be looked down upon heavily by the community.