If you sell it while it's still broken, if it were me I'd pull the hard drive out. I'd either leave it out or I'd plug it into a PC and use an app like Darik's Boot and Nuke to erase the drive. If they reflow it and you have auto login enabled, they will have full access to your account. One way to prevent this would be to go change the password for PSN, but I'd still be very scared. If you do DBaN it, let the buyer know the drive is working but you erased it, they'll have to reinstall the OS to get the system working, if that is their goal.