Apple - Selling a Macbook Pro - removing personal information from it

If you are really concerned with the next owner recovering your files, which can be done even if you emptied the Trash, then you should format the drive:
Open Disk utility, select the drive, select the erase tab and click on security options, then choose one othe following:

I don't know why the sale doesn't include a wipe and reinstall, but I don't see why it shouldn't as a mac always comes with a licensed OS, you just format the drive and reinstall MacOS and you're done.

You can't erase the drive you are currently booted to. You have to boot from an external drive with Mac OS X installed, or boot from the Install DVD.

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.

After creating new user and deleting the old one, at least wipe free space to remove securely your data. However, a lot of data can be retrieved from remaining (for example temporary) files. So that the only secure option is full wipe. And remember to unpair your hardware from any other devices/network services (Bluetooth, MAC address etc).