Hi , you can setup two user accounts on the same Mac . You may want to copy your user from the Mac you are selling to the other Mac so you can continue working and setup itunes sharing for your user . After you want to de-authorize your old mac with itunes so you can sell it .
We can create new accounts in OSX using the Apple menu , System preferences , Accounts control panel , unlock the pad lock , use the + button to add an account . Disable the automatic logon function , restart and logon to your new account . From this account you can setup itunes for your own use .