You just never know your luck on eBay, for good or bad. I bought an authentic small Givenchy bag a few months ago from a reputable seller. Sadly, although it was lovely it ended up being too small for my needs. I gave it to a friend's daughter recently. Good luck with your Nightingale. That's a lovely style!
I also highly suggest that you post on the "authenticate this" thread because Givenchy bags are faked, particularly the pumpkin and nightinggale bags. Posting on there would save you the heartache of a sad fake!