Cost per action/sale methods require that referred visitors do more than visit the advertiser's website before the affiliate receives a commission. The advertiser must convert that visitor first. It is in the best interest of the affiliate to send the most closely targeted traffic to the advertiser as possible to increase the chance of a conversion. The risk and loss are shared between the affiliate and the advertiser.
Another difference is that with affiliate marketing business, an affiliate can sell different products all at the same time. His selling potential is limitless. Even if the products are not related to one another, he can still do affiliate marketing. He only needs to create a separate website to market different products. Unlike with offline middleman marketing, a middleman must only sell related products to maintain his credibility, for who would take a salesman seriously who sells cars and baby products at the same time?