The terms of an affiliate marketing program are set by the company wanting to advertise. Early on, companies were largely paying cost per click (traffic) or cost per mile (impressions) on banner advertisements. As the technology evolved, the focus turned to commissions on actual sales or qualified leads. The early affiliate marketing programs were vulnerable to fraud because clicks could be generated by software, as could impressions.
Go Direst to Merchant Sites: Some merchants choose to handle its own affiliate sales and payments. For example, the Elegant themes affiliate program is run from their website. If you see a product that others are selling then the first place to visit is the merchants website. Most will have an affiliate link for more information on how to join their program.