Pretty much all traffic networks are using the auction model, when selling their traffic. Of course, you can also make FLAT deals with them, especially when going for BIG volume, but that’s not an option for new affiliates at all.
On top of that, with offers going down anytime, caps getting filled at random times of the day and a lot of other factors you cannot influence … I’m not such a big fan of Flat deals anymore, because you can’t stop them at will. These days, I buy bulk of my traffic via bidding auctions.
HOW TO APPROACH BIDDING?
There is a ton of different ways you can approach bidding, everyone will develop their own strategy at some point. Some people love to mess with bids all the time, some set a low one and wait for the outcome, some love to sit in the middle, some at the top … but I don’t want to write a summary of all the possible bidding models here.