Now that you have your lure in place, ie: an eBook, or video, white paper - something of value to your audience, you are ready to make it available for FREE. If you are using an email service such as iContact or Constant Contact, you can get the sign-up code to paste into your html page. Now on your landing page, you would promote this lure or (email harvester). You would have a text link or button leading to the sign-up page. Once submit is selected, their email goes into your database and they are delivered the FREE document. As time goes on and your list grows; you will see your private marketplace expand. It is your job to go out onto the web and spread the word of the availability of your free information. Not only do you benefit from a growing list, but it compliments your expert profile.
The fact that audiences are a diverse group, it would be recommended that you have multiple email harvesters all emptying into the same database. You could segment them to track which product interested them.