I like both the Trade and LFG forum ideas. I know there have been a few times I've either had items I want to sell, give away, or buy, and felt a tad awkward posting about it in this forum. I used to advertise items I won off the AH and offered to sell them at cost (which was usually far below the median AH price) to guild members. I stopped doing it because (1) it was probably annoying guild members and (2) I discovered you can whisper yourself in game (which I then used to take screenshots and record prices of items won on the AH).
I like the LFG forum because it often feels as though I'm intruding on the Raid Schedules forum. I was told that that's what the forum's for, but it still feels weird posting a thread looking for a group when everything else is discussing MC and other high-end instances. Also, it's still unclear where I should post when I'm trying to round up a group of guildies for a quest. A LFG forum would solve that problem nicely.
/shrug
There may be another solution we just haven't thought of yet, but those are my two cents.
EDIT: I still encourage members to download, install, and use GuildAds, but I realize that very few guild members are willing to do so, for various reasons. A Trade forum, while not as convenient as GuildAds, would likely be more widely used.