I'm planning to sell my laptop, and I need to know if I have my bases covered. Kindly confirm please? I have a 13 inch MacBook Pro (10.5.8 OS X - I think it's called Leopard). I do NOT have the installation CD - so I cannot reinstall the OS and thus ensure that I have securely deleted everything from my hard drive. By looking at the o/p from the System Profiler utility, I'm pretty sure my hard drive is NOT a SSD, although I cannot confirm this.

The reason I can't directly use the "Erase data" option under Disk Utility is because the 'Security Options' (as well as the volume format) button is greyed out. Volume format currently reads 'Mac OS Extended (Journaled)

I'm planning to securely delete ALL data on my mac using the following steps:

1) Using Disk Utility, I click the button titled "Erase Free Space" using a one pass method (or if I have time 7 pass method) 

2) Once the above operation is complete, I plan to use 'secure rm' or 'srm' to remove all files and applications on my macbook.

3) Once step 2 is complete, I re-run "Erase Free Space" using whatever method I used in step 1 above (in order to remain consistent)

Once step 3 is complete, I assume that my macbook has been formatted and all data securely deleted. Could you kindly confirm whether this assumption is true or not? If not could you please point out steps I need to add to my existing process?
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.