How strange, I just posted about my Giselle in the thread about green bags being a hot item for spring. So my Giselle is a few years old and aquired at Saks. I do remember LOTS of faux Giselle bags on Ebay at the time that I purchased mine. I took a look at the ebay auction that you listed, and based on the pictures provided, it looks exactly like my authentic bag. However, I have really no experience authenticating her bags, so I don't know what specifically to look for in terms of authenticity. I rarely purchase bags on ebay because of all the fakes.
I'm a newbie here, so I don't know if there is a better place for this authenticity thread, sorry.......
Best of luck!