By impression - every time your banner appears you pay. By sale - every time someone clicks on your ad and then buys your product, you pay a commission. There are also hundreds of Free Banner Ad Exchanges. Try Linkexchange . Of course running around to web sites buying space is cumbersome, time consuming and frustrating.
Many sites handle their advertising through a third party.
You can buy space on hundreds of sites at once. Many of us need sites that are more modest than this. This program is well established and has 1000's of webmasters - many of them running small or specialized sites. Northern Lights Internet Solutions Directory Sites Find out about advertising on Northern Lights sites. This is a great program with lots of affiliates.