Wiping Sensitive Data Without Reinstalling OS?

I'm lost. Did the new buyer asked for Office to be installed?

I'm not completely sure, but there are two ways to delete the trash in a Mac: "empty trash" and "secure empty trash". The later deletes the files in a way for them never to be able to recover them. The later just puts them as "available" space to replace.

Anyways, I wouldn't sell a computer without making a clean install, unless I knew the buyer. Why risk it?

You can create the new admin user account, in Apple Menu > System Preferences > Users & Groups (Users & Groups name varies by OS X version), then from that account delete the old one and choose the option to remove all user files and settings.

Then you can go into the hard drive and delete any files you may have put in the shared user folder or the root level of the hard drive. If it is still listed in the Users folder you can delete your old account entirely.

Once that is done you can go to Boot Volume\Applications\Utilities and open Disk Utility. Click your drive and go to the Erase tab. From there select the option to Erase Free Space and choose the most secure option then erase. This will take quite a while. I would go to the Energy Saver pane in System Preferences and set your computer to never sleep to avoid interruptions.

This won't delete all references to your name. Any registered software will still be in your name. There may be other references to you, your address and other personal info scattered about the System, Library and main hidden folders. The only way to be certain is to replace the hard drive, destroy the old one and reinstall OS X.

However what info does remain would require someone very knowledgeable to dig it out. What remains is likely much less than personal info than can be found in a few quick Google searches.

There are three ways to erase data on your computer.

1) "Empty Trash". This action removes the marker on the drive which indicates the location of the file. Provided the file is not subsequently overwritten by another file, using easily available software, the file can easily be recovered.

2) "Finder > Secure Empty Trash". This action overwrites the data with random characters which, theoretically makes the original data impossible to recover.

However, nothing is impossible! - If you are really paranoid:-

3) "Disc Utility > Erase > Secure Erase". This is similar to (2) except that Disk Utility writes over the erased files’ data, so it can never be recovered. You can choose to write over the data once, 7 times, or 35 times. Writing over the data several times is more secure than writing over it once, but takes longer.

Hope this clarifies things for you.