You can see who is using them -
http://www.adbrite.com/mb/dir.php
You probably have a good chance going direct. They're using Adbrite to sell ad space, and if you go direct, they don't have to split it with them.
"Interesting, where can I go to buy traffic?
Can you give me more information about selling ads on the site?"
With them, you can find sites you want to advertise on, the different options etc.
You can also sell advertising thru them. Put some code on your site, where other people's ads can show.
I've only done the flat rate stuff. Haven't tried per click and I think they have per impression also? Not sure
Actually, if you go to the very bottom of my site in my signature, you can see it in action. I had a link to buy an ad and had it set to where I manually approve but just to test things out, changed it to auto approve this morning and ads are showing already.