You should wipe and reinstall. Reasons:

- personal info. Anybody can attach your Mac in target disc mode and copy the whole HDD for later analysis. Even if a file is removed by emptying the Trash, or via the "rm" unix command, there is a possibility to recover it (because it deleted only the reference to HDD sectors and not wiped the sectors).

- applications - If you have any applications, you probably want to use them later too. If you sold the MacBook with them, the buyer will use them too - double install usually means disabling your legal activation key.

- with clean reinstall, you delete everything - for example `/Library/Application support` too, where much applications saving some infos (non personal, but anyway).

- another example - the `logfiles` in the `/var/log` directory. Here are tons of informations that can be gained from them (for example mail addresses) and so on...

- erasing the Home directory is simply not enough (see above).

- as @Petruza already said - the only way is to wipe the whole HDD and reinstall the OS X.