Buying used upgrade disk

Q u o t e:
Is it permitted to buy and use WOLK from a friend who no longer plays?

To be clear, once an account has been upgraded using the Authentication Key contained within the box, it cannot be reused. In other words, purchasing a used DVD from your friend would allow you to install the client, but would not allow you to upgrade your account. Accordingly, you will want to purchase a new copy of the game at a local retail provider or via the direct online upgrade feature.

For more information, I encourage you to review the following page:

http://us.blizzard.com/support/article.xml?locale=en_US&articleId=20499

Let me know if you have any questions.