Great question thanks!
First, I'd say that as long as you aren't try to promote or sell any specific product or business to which you have a personal affiliation or means to profit by, you would not be in violation of any MMP rules or guidelines.
I especially like the idea of turning to the community when in need of a particular coupon. As you say, you never know when someone else may very well have exactly the coupon you are looking for. And, obviously, such a request can be easily posted in question form, e.g. Does any one have a Brookstone promo code I can use?
As to posting about a coupon that you don't need but think someone else might -I think that would be fine as well. In either case, I would probably be inclined as moderator to post a brief disclaimer generally stating that any such transactions and offers are strictly between members and not officially MMP endorsed or facilitated. I would probably also add that cash or payment for such items is against MMP rules. Free giveaway is fine, but selling would be a violation.
Anyway, those are my thoughts. Overall, I think it's a great idea, thanks again for suggesting it.