$800 seems a bit steep for that system. Used equipment does not fetch any premium at all. I think the only thing that makes the system potentially worth it is the software and the monitor as mentioned above. I would make sure you get all CD media and product keys for Windows 7, Office, and Photoshop. Without the software keys, I would say about $600 with monitor.
It doesn't seem like a bad deal. The i5 2400 is a generation old and we're right on the cusp of Haswell. Same for the GPU. That said, though, they're both pretty good performers and the software alone has pretty high value. $100 for a Windows License, $100 or so for Office 2010, and another insane amount for Photoshop CS5.
Whether you credit the build for those licenses depends on if they're all legitimate and if that matters to you. If you don't mind using cracked software, then I wouldn't credit the build as much because you could just attain the cracked software yourself for no additional cost. If you do value legit software licenses (I do), then I would say it's definitely worth it if these are legal installs.
The included monitor is also great for that price.
Why is he using 3 sticks of 2 GB DDR3 with a z68 motherboard? You should be running two identical sticks with that chipset. There's no triple-channel support for Sandy Bridge.
I wouldn't give any credit for those hard drives, either. What 250 GB drives is he using? Can't be as fast as modern 500 GB or even 1 TB platter drives from Samsung/Seagate and Western Digital.
Even despite those knocks, though:
Definitely worth the price for processor, software, and monitor.