I would say it isn't authentic (though it looks mighty real!) for the following three reasons:
1). It's a one-day auction - usually sellers do this because they don't want ebay to pull their listing
2). The bidder list and his feedback information is private -- why would he/she do this unless he/she has something to hide?
3). Finally, he/she doesn't even mention that it is authentic (not that claiming authenticity even amounts to a hill of beans on Ebay, but the fact that he/she isn't even mentioning it really means a lot! So I have to give that person some credit for not trying to claim authenticity)
Nope, it's not. For the reasons that shoppegirl mentioned plus one other that I don't even like to mention since the fakers read these boards. I guarantee you that bag is fake though.
Remember, there are NO deals to be had on ebay for a "hot" bag! There are exceptions, like a seller selling her own used bag, etc. but for the most part, don't think you'll get lucky finding a $1200 bag for under that price.