Never agree to sell your list! Purchased mailing lists are email addresses that have been bought from someone else for a price. Many companies contemplate buying lists as a means of gaining access to a particular group of email addresses quickly, but consent is not transferable and using these types of lists is not good marketing.
In addition to subscribers on the purchased list having not opted in to receive emails from you, they are likely going to flag your email as spam, and you have damaged your marketing relationship with them before it has even started. Using purchase lists will also directly affect the delivery of email campaigns you send to people who have opted in to receive your emails.