I don't think it's real either... I was looking for Kooba bags a while ago and did see this seller on eBay that you mentioned. She has sold a number of Siennas and all a couple of hundred below retail. She had a Chloe Paddington listed at one time which had a starting bid of $399. Now, not that a low starting bid is an absolute indication of a fake, but it should ring some alarm bells at least.
Also, the pics she uses for the Siennas looks like her standard pics for all her Sienna listings, so not pics of the individual specific bag you're getting. Again, alarm bells.