One thing to note on used computers is their age and past use. Just like a car's mileage, it will come in to play with the life of the parts. Especially if the previous owner was a gear-head always trying to crank out more power (like a gamer over-clocking his processors).
For that reason alone I'd recommend against buying a used machine unless it's from some one you know personally.
Extremely good specs on that one on paper. Really depends though
1. At how much you're getting it ?
2. How old is the system? &
3. How much has the system been used and at what settings?
If used sparingly/moderately at stock or minimal OC, with decent cooling for a 2 year old PC, 300 may not be be entirely bad and 250 might just be a great bargain especially, if the system has been well cared for. Open it up and the tell-tale signs will all be there.
Thanks for the replies, guys. Yeah, he knows the person, and is buying in the US. Kind of difficult to say, though, as he's not really actively looking for a computer but his friend is upgrading and both sides want a "fair trade". Personally he wants to run games that are coming out (Diablo III, Skyrim at most, and not at any insane settings), and if it is possible to upgrade the video card or something to run even later games down the line that'd be good, too. But mainly he's just looking for a price range to offer and hit.
Given what I've seen some of those individual components go for on places like Ebay it's not worth it, unless it's in the region of 200 dollars or less. GFX isn't DX11, cpu will need overclocking to get anything good out of it (they do overclock well however).
I've seen q6600 go for 60/70 dollars,
ddr2 ram is also really expensive (heaven knows why), and those I imagine would cost 2x more than a brand new set of faster ddr3's.
My advice is that if you can squeeze it for 150 dollars or something that would be considered rediculous, it'll be ok for a while but I expect you'll be updating in a year or so, then you could use it for a home server or a doorstop.