Originally Posted by BoeingBoy

While there are brokers and the airlines do dump unsold tickets to them (usually in the 24-48 hours before departure), the big online GDS' aren't brokers. Just like your local travel agent, they sell from the airline's available inventory - if the airline is sold out in a particular fare bucket the GDS' can't sell you that ticket.

For some reason, US' website is rarely shown as having the lowest fare available. I use Kayak, which doesn't sell tickets but merely consolidates the listing of fares from various siites (both airline and GDS) and provides a link to the lowest fare provider, and US' website is almost never listed as the lowest source for US tickets. At one time it was because of the $5 hidden fee for booking on US' site which made the big GDS' $5 cheaper - maybe that fee is still there.

Jim