I'm a little curious as to why anyone would buy a Q-type processor (Intel's pre-"i" series quad-core processor) now that virtually all anyone is selling in the non-budget Intel line these days are the i3, i5, and i7 chips. You can get an i7 chip from NewEgg right now for considerably less than the builder allegedly paid for this Q9550 last year.
The 4870 isn't a bad video card, by most standards, but the ad cagily doesn't note whether it's the 512 MB or the 1 GB model.
That the ad's poster touts spending upwards of $1,400 on the system and is selling it for $650 sends up huge warning bells in my mind. Especially on Craigslist. "Lifetime warranty"? Good luck following through on any of that.
If everything is on the up-and-up, the system described would probably play most games that are out now reasonably well, but will start slipping badly when the next console generation comes out, and have very little upgrade path (the Q processor in particular means the motherboard's upgrade path is likely just about nil.)
Personally, I'd advise against it.