@noahl, I used to use a few discount sites until I realized, like @anotherday, that only a portion of the tee times are made available to the brokers for resale—for GolfNow that's Golf Channel Solutions, while GolfSwitch backs GolfHub (each also sells through many affiliate sites.) Since that portion is sometimes only what the course thinks it can't sell normally, e.g., by phone or through their own online booking site, I find I really have to talk to them to find the best fit for time (and at I've never had a problem getting, at the very least, the same deal as shown online when the time is the same.) It's definitely a problem still looking for a great solution. I think that great solution will have to combine complete tee time inventory with planning and social tools that increase the connection between players and all courses—not just the 15% or so that have signed up with a middleman that further disconnects player and course.
If I may make a self-serving suggestion, how about also looking into the startups just now coming out with round planning web sites and apps that also help, from a different angle, people get the best tee time for their round. For example, Teeboxer (full disclosure, that's my creation), ClubDivot, TeeTimeBuddy, and FullFoursome. And, there's another site you might look at that's doing interesting things by opening private club tee times to non-members: Boxgroove.
Regardless of whether that's too far off track for your story, feel free to get in touch if you want more background info (not that I'm a font of it, but always glad to help where I can.)