There is a blurb in the guidelines about trading as it applies to profiteering.
I don't see where it applies to simple trades between members and a forum for that purpose could be done. It depends on whether the users want it and if the admins have reasons to ban something like that - reasons that arent known currently. As it stands, it is simply disallowed - no explanation given.
Im not all that bright and there may be problems with such a forum I dont readily grasp, from a moderation standpoint. Too, advertisers might howl or suppliers mutiny, stuff like that. They figure if we're trading outside their circles, then they are losing profits. In that case, it's back to profiteering; it just depends on which side of the profit fence you happen to reside...
But, it's doable and Ive seen it elsewhere. As always, getting the right people motivated is the main driver towards making it happen.