It depends on what you're selling and what kind of email you're sending out. If you're sending out an html email, you should be able to track click throughs and ad views.
Also, remember, simply using the threads admin email function will still allow users to unsubscribe to the admin emails.
I certainly would not sell my email list...EVER! Let them pay you to send out an email to your users, don't give them the list! If you do, you will lose a lot of members and you sold them out, so trust is gone as well! You should be representing their best interests.
