I guess one could do one of two things:
1) As soon as OS X is installed and starts asking for login info for the new user press command-Q and shut down the computer. Then when the next owner gets the computer it will be as though it was brand new from Apple. The only drawback is that the usually installed iLife (or iWork) will not be included.
2) Enter a temporary user account and install iLife and/or iWork. Then instruct the new user to create his/her own user account and delete the one created to install OS X.
Is there another way?