With any business it all comes down to how much consistent effort you put towards the business. Just putting an affiliate link on your website doesn't guarantee sales. You still have to direct people to go there and give them a reason to buy so the products offers and the informaiton on your website is important.
The company I work for has an affiliate program to sell our books. Custom links simply send people to a special web page (created for free) to buy the books . All the credit card processing, and shipment of the book is taken care of on the website. AFFILIATES ARE NEVER REQUIRED TO BUY ANYTHING. Commissions range from 8-40% depending on the book.
Find out about the most recent best seller here: www.stressout-book.com
and the affiliate program here: http://bit.ly/2gm6N