There's no way you could have known. The listing shows pictures of an authentic bag - not the one you received. Compare the creed on yours to the picture in the listing. I think that is proof for a SNAD case.
Here's the listing: http://www.ebay.com/itm/221257034225?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Becca, one thing you need to do right away is make copies of all the photos from the original listing - that way if the seller removes or deletes the photos you still have proof for a SNAD claim that the bag you bid on isn't the one you got, and there are enough differences between the two to convince you that the one you received is fake. Save a copy of the entire listing too.