It also means we have time to really look at each ad to make sure it's a fit for the audience or if there are ways to make it better.

But these aren't that much of a problem in practice, really. See how affiliate marketing works: it's CPA for all intents and purpose masquerading under a different name. The biggest difference might be that the vendor will contact you, rather than make some kind of bid, if he thinks you've the right kind of traffic.

If your audience is interested in a particular product, you quickly discover it through common sense and trial and error, as you can normally see the click-through rates.

Also, a big ticket item with a low conversion rate sometimes earns better than a smaller one with a high conversion rate. As such, maxing out your ROI is really a matter of seeing what contributes to your bottomline.