Never agree to buy a mailing list or to sell yours! Purchased lists are email addresses that have been acquired from someone else, such as a broker. Many companies contemplate buying lists as a means of gaining quick and easy access to a particular group of email addresses. The issue is that consent is non-transferable, which renders using purchased lists illegal and not a valid marketing strategy. The CAN-SPAM specifically states that it is illegal to sell (and so, conversely, purchase) email addresses.
Subscribers on a purchased list did not specifically consent to receiving emails from you. They will likely report your emails as spam, and you will have tainted your business relationship with them even before it has begun. Additionally, the use of purchased lists will impact the delivery of your email campaigns as a whole, thereby also affecting those who have consented to receiving emails from you.
A rented list is similar to a purchased list in that the sender has no existing relationship with the members of this list. These lists are typically used to send a one-time email campaign aimed at quickly reaching a new audience. When renting a list, the marketing manager will never actually get to see the list as such. The marketing manager will provide the campaign’s content to the list holder, thereby resulting in the marketing manager being at the mercy of the list holder with respect to the validity of the list (consent) and the sending of the campaign.
Members on a rented list may feel that you are using bad marketing practices and flag your campaign as spam. This practice could, therefore, result in bounces and/or a high rate of spam complaints, thereby tarnishing your reputation as well as that of the list holder.
Regardless of who sends the email, if the content is yours, you are responsible for what others do on your behalf.