The PC that's being advertised would have cost more than $1100 brand new (I know, that's too much for the components, but this is Alienware after all, so it's not that surprising - plus you're paying extra for the relatively small form factor).
For $700, you certainly couldn't match the components if you built a new system yourself. but then you probably wouldn't want to as it's a bit of an unbalanced system; an i7 4770 combined with a 192 bit GTX 760 (a gimped version of the normal 760) isn't the best match-up for gaming. An unlocked i5 with a 770 would be much better.
A key thing to remember though, when it comes to comparing this to a system you could build yourself, is that this already has Windows, which is a ~$100 extra expense on top of the cost of components; $100 out of $700 is a big percentage. So really, other than another used computer, you're not likely to see a better deal for the performance this offers, even taking into account the unbalanced nature of the build.
I'm not going to recommend you get it or that you don't get it though, as that's something only you can decide, as it's a question that has a lot of factors to it: (are you okay with a used PC? Can you be sure the previous owner didn't wreck it? Do you trust them enough to buy it from them on Ebay (thankfully Ebay has pretty robust anti-fraud measures)? Does the fact the small case potentially limits upgrades matter to you? etc, etc...). There are a lot of questions you need to ask yourself, and some you potentially need to ask the seller, and with that I wish you luck.