I couldn't have said it better than DJ Waldow:
"The problem with list rental is that the subscribers are not nearly as engaged as those who find you directly and ask specifically to be included in your email marketing campaigns. To me, that is the biggest difference. From what I’ve seen, list rentals tend to have much lower open, click-through, and conversion rates. In and of itself, lower rates are not a cause for panic, but a damaged reputation is. In my experience, emails that are sent to rented lists have more complaints, are more likely to be marked as junk/spam, and have a higher unsubscribe rate. This is where you can run into issues."
If you do decide to go down the list rental route, just make sure to check how the opt-in was obtained (don't just ask, check it for yourself!) and how often the people on the list receive emails. Make sure that the sender of the email is the list owner and that the email starts by saying why the recipient is receiving this email.