The CDs don't do the purchaser any good without the account, so yes. The CD # became invalid the moment you created your account.
People sell their account, but that is not endorsed by Blizzard and it is probably against the Terms of Service you click past when you first install the game and after each new patch.
You can download the client from some places I think. I know I never even installed from the CDs as my open beta client was simply patched to the retail version. I created my account with my CD key, pulled the manual out, then the CDs went right back into the box and on my bookshelf.