Hilton HHonors - Booking on Hiltom.com vs. expedia, travelocity etal

I was looking to book at the HHV in August. The dates that I put into the hilton.com site came back with "no rooms available."
A check of expedia, travelocity and some other sites, showed rooms for immediate booking.
I was under the impression that the booking sites acted like an agent, taking a commission, and booking from an available pool of rooms. Would they have a different pool than Hilton? OR, do they buy (guarantee) the rooms from Hilton, and then turn around and sell for whatever the market will bear?

<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Jakester:
do they buy (guarantee) the rooms from Hilton, and then turn around and sell for whatever the market will bear?</font>

Yep, you got it right. Certain travel consolidators buy inventory from hotels with huge volume discounts and then sell it for profit.