I want to track outbound links to see if they place an order

I am making a site which has outbound links to the products which I am reviewing.

I want to be able to track the outbound links and see if they place an order and how much the order is for.

Essentially, I want to be able to create affiliate links for small businesses that do not have an established affiliate system set up.

The external sites usually employ a 3rd party booking system. How can I track how much business my outbound traffic is generating?