Well, let's break this down:
MOBO: (~$80) Not bad. Plenty of room for expansion, and you can use the second PCI-E 2.0 for Cross Fire/SLI or (in my case) an SSD. I'll base it's value on the MSI 797 PC Mate and nix a for age.
CPU: (~$125) It's an old, but faithful Sandybridge. Yeah, I know it's from 2011, but my Ivybridge i5-3570k hasn't bottle necked me yet, so a gen prior isn't going to be too big of a deal. On top of that, that K means it's unlocked and ready for overclocking! Based on auction prices (it is old and a bit more lets you get a current gen anyway).
GPU: ($90) Okay, my AMD/ATI bias will show here, but I'm just going by age and current day options. A decent review shows this card is comparable to my GTX680, which is NOT bad at all, considering I've only decided to upgrade recently from the very card due to wanting a better VR experience. That being said, the cost it came out in 2013 was $190. While this VERY card's availability may not be the best, you can easily find GHZ Editions from other companies on auction sites for around that price tag.
OS: ($150) Okay, this one I have to admit is going to be the oddball part, since Windows 7 is slowly being phased out (Typical). While you can find OEM copies of Windows 7 Professional, Ultimate is much MUCH harder. I'm not sure WHY a normal person would need ultimate, as Professional is typically more than enough (and is seemingly the community's pick of OS grade). That being said, Windows 7 IS still available, assuming the key used IS LEGIT!
PSU: ($150) This is STILL available on NewEgg!Based on retail price (and free shipping).
Case: ($145) This ALSO IS STILL available on NewEgg and doesn't look half bad! Of all the components so far, I think these two are the MOST recent. If this is the HXi varient, then I think it is modular as well! Make sure the cables are included. Based on retail price (and free shipping).
SSD: ($45) Okay, let me be real with this one: I KNOW this particular SSD is pretty well known, but the cost of more recent SSDs of the same size has come down since that came out. Price is based on price tags of identically sized SSDs.
RAM: ($55) NewEgg still sells this. This person REALLY loves his Corsair stuff. No complaints, honestly. Based on retail price.
DVD Drive: ($20) It's a DVD drive. 'nuff said.
Overall Value: ~$860
Even if I nixed another hundred or two for wear and tear, the $600 price he was wanting is not bad, though the drop to $400, considering he wants to 'upgrade' makes me scratch my head, since $400 isn't going to get a lot. Considering the case and PSU are both VERY good, I'm not sure why one wouldn't keep those two things and just sell the remaining stuff as parts for the asking price (plus, having a Windows 7 key on hand). If the deal is real and legit, it isn't bad.