Frankly, I think the buyer is pulling your leg because if she believes you sold a fake, 1/2 price isn't going to magically make it authentic. And $89.50 would be way too much money for a fake. So IMO, the buyer is just looking for a lower price.
Based on the pictures in the listing you've posted, the bag appears to be fine although there should be a clear, head-on picture of the creed. We always recommend a photo of the creed in every listing.
If this is the item she bought, I can understand her concern because there aren't any other pictures EXCEPT the stock photo.
Does the buyer say why she thinks it's fake? Please refer the buyer here and perhaps we can address her questions and ease her concern.
I do have a few comments regarding your listings.
1. You shouldn't be using any of
Coach's copyrighted stock photos in any of your listings. It's unethical and illegal to do so plus it prevents the buyer from seeing the actual item.
Since you clearly own a camera and you take good pictures, you should replace any stock photos in your listings with your own picture of the same view.
2. Listings should always have a picture of the creed, hardware, strap, stitching, etc. Some of your listings have great pics and others are lacking.
i really appreciate it. 
