selling online ad space?

[marcus]

You should charge based on the page views of your site (you'll find those in your Google Analytics account, if you have one). Your client will pay a CPM or cost per thousand views. Depending on the quality of the traffic coming to your site, you can charge anything from $0.10 - $3 or more CPM.

So let's say you're getting really high quality traffic in the "work from home" niche (people will pay good money for this traffic). And let's say you agree on $4 CPM. Your website gets 50,000 page views per month. Your client would pay $200.

You don't need software, just insert their banners where they want them. And PayPal would work fine for processing the payments.