If you look at the Alienware website, a 250GB HDD isn't even an option with with the i7-2617/GT540 video card. Therefore it's either at least a 320GB HDD, the 256GB SSD. or he bought it and downgraded the HDD. Since he said 250GB, I'd place odds on it being a SSD and he just didn't clarify on Craigslist.
It's a good buy if it's in good shape either way, but with an SSD it's a STEAL. The minimum configured price for that processor/video card combo is $1199 assuming the 320GB HDD. Either way you are getting 50% off at least, and he seems willing to let you inspect it very thoroughly.
What can it hurt to take a look?
And if it really is a 250GB HDD, call him out on opening up the system and downgrading parts without telling you. Maybe he'll lower the price more or offer to give you the HDD/SSD it came with so u won't back out/give him bad feedback.?