Hard to judge really. Can't really say much about it, since I don't know what an Alienware compute costs over there. Don't know much about United States currency.
Well, over here we have an Ebay-like website, and on the news one time they showed some people who had been duped into paying for something, then not getting it. So maybe that's what that guy is planning to do? Then again, I doubt there is a way in telling.
I would be wary. If you look at their other items for sale and at their feedback rating. You will see that they have purchased a lot of low priced and they are selling $$$ items currently, either they came into a lot of excess $ or they hijacked someones house/truck would be my guess.