If you wish to sell digital products online e.g. eBook, MP3, images etc, you can use a well established 3rd party service like Payloadz. To do this follow the steps below:

1. First you will need to open an account with Payloadz (which you can do for free)
2. You can add your products, pricing and any other required information on their site
3. When your account is set up go the Account page
4. You can then copy the supplied snippet of HTML - You will have option to add button or text link.  
5. Paste the code into your HTML Snippet widget
6. Save your page