If somebody offers to sell you a bulk list of 'opt-in' email addresses, you're almost certainly getting a spam list. What you're going to receive from them is a list of addresses belonging to people who have not agreed to receive messages, or a list that's been mailed to so often that it is no longer useful to other mailers. To put it another way, whatever they might have opted into, it wasn't mail from you.
As a sender, it is your job to confirm that your list is truly opt-in. Where do you begin? Doing your homework saves headaches later – headaches driven by poor results, spam complaints, blacklisting, wasted time and money, and bad PR. In other words this is not a responsibility you can shirk by saying 'I have no way of knowing how accurate or authentic these names are' or 'They guy from the list rental organization says it's opt-in'.
It's your responsibility to know about the names your communicating with. Sending unsolicited commercial email will bring a number of problems to your email program, to mention a few; Major DNSBL listings and ISP blocking, recipient complaints, and a negative sender reputation