As I see it, the problem is the most of the exchanges or rentals being listed are location specific. I think the solution is more of a points for credits exchange and vice versa.
I have suggested something similar to Brian for TUG. That a points specific exchange category be added to the TUG marketplace. It would allow points owners to exchange reservations. You would advertise your membership network, available points to exchange, and what network you would like a reservation in. Then you would work out the details with the other party.
So your ad would be:
WorldMark member willing to exchange up to 14,000 WM Credits for Hyatt Key West reservation for Jan 2010.