If you can only download it once, you can't really reinstall everything (deleting anything that could link you to the laptop) and keep Office.
The best way to sell a laptop is to completely erase the hard drive and reinstall OS X (without setting up OS X). Then clean the outside off as nice as you can and prepare it to sell.
Really the only way you can do it in your case, keeping Office, is to make a new account, back up all your personal stuff you want to keep onto an external drive, log into the other account, and delete your account.
Though, the only thing is that it may leave some traces linked to you.