Oh, forgot to add:

Fares available on Orbitz might not be bookable via telephone or gate agent. I have had the experience a number of times where the computers at United (also Apollo) show no availability, but Orbitz does. Don't know about Expedia or Travelocity, et al, but I would assume similar in many cases.

Essentially either some seats are being set aside for these services or they are buying them ahead of time for use (virtual consolidating?

In these instances, you are SOL regarding your discounts. I have never been able to get these done because purchasing is immediate when using these services and furthermore the resulting tickets/reservations are travel agency - hence not touchable by United.