You should wipe and reinstall. Reasons:
personal info. Anybody can attach your mac in target disc mode and copy the whole hdd for the later analyze. Even if a file is removed by emptying Trash, or via "rm" unix command, here is a possibility to recover it. (because deleted only the reference to hdd sectors and not wiped the sectors.
applications - If you have any applications, you probably want use them later too. If you sold the macbook with them, the buyer will use them too - double install usually mean = disabling your legal activation key.
with clean reinstall, you delete everything - for example
/Library/Application supporttoo, where much applications saving some infos (non personal, but anyway)another example - the
logfilesin the/var/logdirectory. Here are tons of informations what can gain from them (for example mail addresses) and so on...erasing the Home directory is simply not enough (see above)
as @Petruza already told - the only way is wipe the whole HDD and reinstall the OS X.