Here is an interesting idea. Instead of everyone spamming trade channel hoping to get a bite, have two special channels, /buy and /sell, that you wouldn't have to be subscribed to in order to post in. I'd prefer to start the game not subscribed to the trading channels, with a little pop up telling you how to join them. That way, sellers would subscribe to the /buy channel only and post "/sell (item being sold, price)" and buyers would only need to subscribe to the /sell channel and could post their needs via "/buy (item you want, price)". This should reduce the amount of chat spam people have to sift through.
I prefer a trade channel to an auction house due to the human interaction, but not if it is this difficult to follow the chat.