1. Visit https://www.google.com/settings/plus.
   
   
2. Scroll down to a heading called Profile (Its almost at the bottom of the page)
   
   
3. You can select "Allow people to send you a message from your profile" and/or "Allow people to send you an email from your profile". You also should set the option to Public so that way anyone can notify you of an listing/item you had for sale (see example below).
   
   NOTE: This is a screen shot from my personal account and I have elected to have only the email option as I prefer to get an email instead of a posting on my profile page.
   
   
4. Now you are finished.