Supposing I buy Xenforo today. Supposing for the next 2 years I did not renew the license.
That means if I buy today, I have till September 22, 2015 to renew. Supposing I do not renew until September 22 2017.
What happens? Do I still pay $40 to renew my license in 2017?
Be aware that if you do purchase a license from someone else, you cannot sell it again if you decide you no longer need it. They license can only be transferred once (which is what will happen when you purchase a used one).