Legit? Probably.
Good quality for the price? No..
For instance, the top left linked build with the Corsair 540 Air. $486.
The case retails for $130. Maybe they get them for $95 in bulk. Not cheap.
That leaves under $400 for the rest. Where did they cut corners?
Oh, here:
Onboard video
Crap PSU
At that price, go cheaper on the case and better parts.