Let's see... nobody but Alienware sells Alienware computers... so, if these guys are offering one for half the price Alienware does, there's really a very short list of ways they could do that:
1. They're losing a ton of money on each one (yeah, right).
2. They're selling stolen Alienware laptops.
3. They're not Alienware laptops at all, or
4. They don't intend to ship anything after they get your money.
Factor in that the only method of payment they'll accept is a Western Union transfer, up front, and that the whole operation appears to be run by one guy in Bulgaria who seems to make an awfully big deal on his site about how it's *not* a scam...
Well, ever heard the phrase "you get what you pay for?" My guess is that, with this guy, you wouldn't even get that.