Disk Utility – 3 Pass Erase

Not so much that it's not necessary...

I'm a couple years late to the party, but it might be worth pointing out that Apple (who has now entirely remove "Secure Erase" options from the Disk Utility app) hasn't really removed the option because it "isn't necessary" — according to its El Capitan security release notes, they did it because they can't guarantee a secure erase:

Description: An issue existed in guaranteeing secure deletion of Trash files on some systems, such as those with flash storage. This issue was addressed by removing the “Secure Empty Trash” option.

Glenn Fleishman, gives a good overview of this in "How to replace El Capitan's missing Secure Empty Trash." The fact that, currently, the only DoD/NSA approved SSD sanitation procedure is smelting or shredding the drive into a fine powder somewhat echoes the challenge in being able to wipe a drive, for sure.

It is pretty difficult to recover data from an SSD...

As Trane Francks explained, recovering data from an SSD is, by default, pretty difficult. The data isn't necessarily encrypted, but it is distributed data over many locations in order to perform as few writes as possible to a single location (both for performance and for drive longevity). So once data is deleted, find the place a file used to reside in is like putting together a multi-million piece jigsaw puzzle (all before any garbage collecting the drive may decide to do). It is possible to recover files from an SSD, but this usually requires a lot of extra effort.

To make it hard for recovery tools...

Encrypting a SSD with any kind of suitably secure key, and then erasing that key, makes it virtually impossible to recover any data. This can be done, on a Mac, by enabling FileVault, booting into recovery mode, unlocking and then deleting the drive with Disk Utility.

If you're just looking to make sure stuff is securely erased without nuking existing data, you could try using the diskutil terminal command — the command line version of DiskUtility, wherein secure erase options have not been removed:

sudo diskutil secureErase freespace 0 "/Volumes/[Disk Name]"

This should attempt to write and delete a couple tempfiles which will fill up the entire hard drive. In doing so, every available space should be filled and then cleared.

Good information on all these options can be found in "How to Securely Erase a Mac SSD".

Also, you can always try to run some data recovery tools to see if there is data that is still immediately available.

You will not be able to erase the entire contents of the disk, format/remove the partitions (securely or otherwise) while the volume is mounted. Because of this practical limitation, without having your original installation disc from which to boot into Disk Utility, you're stuck with a system that is in a vulnerable state to sell. While you can create a throw-away admin account from which to securely delete your user profile, OS X still splatters information, such as network connections, all over the place.

As Buscar correctly stated, the resale value on what I would I imagine is a circa 2008/2009 system is low enough already WITH all the original discs. Without them, the possibility of selling is fairly low. The biggest reason for this is that you need the original discs for running diagnostics. Apple Diagnostic software is mated to the particular hardware.

Although Apple no longer supports these systems and, therefore, does not sell replacement DVDs, you can purchase recycled installation DVDs from ifixit.com. I purchased original Leopard install DVDs from iFixit for an Early 2008 MacBook4,1 that was gifted to me without discs. Highly recommended.

With the correct DVDs, you can securely erase the HDD and let go of the system with complete confidence. Alternatively, you may consider simply replacing the HDD in the system with a new one and destroying the one you've been using. Your out-of-pocket cost is probably around the same, but the original DVDs would likely net you a higher resale value.