The actual purchase of a mailing list along with the rights to mail it directly. Permission can only be “sold” if the subsequent mailings continue to match the frequency, brand name, content, and “from” of the past owner’s mailings — and even then this is a somewhat shaky procedure on the spam-front. You are in effect buying a publication, and not just a list.

The 3 Things To Avoid When Emailing Your List
When you decide to have an opt-in list, it is not just a matter of sending your subscribers your promotional newsletters or catalogs.