People are concerned about the sale of email lists because of privacy issues. Many people do not want to receive spam online, just as many people do not want telephone calls selling them things. Spam is unsolicited email, usually sent to large numbers of people at once. For people who have limited space in their email boxes, spam can fill their boxes and prevent them from getting personal email. Spam is also used for identity theft.
The laws on selling email lists are not straightforward. The United States does not have a law that specifically forbids selling email lists. It does have a law regulating spam, the CAN-SPAM Act of 2003. If you sell an email list to someone you know is using it for spam, you might be guilty of conspiracy. If you misrepresent the nature of the list, you might be guilty of fraud or conspiracy. Other countries, such as the United Kingdom, do have privacy laws that apply to selling emails without the permission of the people on the list. Those are the Data Protection Act and the Privacy and Electronic Communications Regulations. If someone on your list lives in a country with such a law, you could be in violation of that law by selling lists that contain addresses of people who have not given you permission to send them email.
There are two types of email lists: those you have collected without the permission of the people on the list and those consisting of people who have opted to receive email. Opt-in lists are created when people check those boxes you see at the bottom when you sign up for a new service or to receive something online. A box might say something along the lines of "Would you like to receive notice of special offers by email?" Anyone who checks one of those boxes has opted-in to receive the email described. It is legal in all jurisdictions to sell opt-in lists.
The best way to avoid legal problems in this situation is to collect email addresses only from people who opt-in for the kind of email that will be sent by the list purchaser. You also need to be careful not to misrepresent the nature of the list in any way. The considerations for online users are different. If you do not want your email address sold to other vendors, think twice before you check an opt-in box.
A valid opt-in list can be valuable. If the people on the list really are interested in receiving the kind of information they will be sent, then they might benefit from increased information. For example, if someone opts-in to receive information about travel discounts, he might be happy to get information and discounts from three new sources. These lists are also beneficial to the purchaser, who can reach people really interested in her information.