I would say you have two options here, of which i would suggest doing the first:
* ask her to return it to you if she is not happy with it. If you are sure that it is a genuine Chanel, then I would suggest you take it to a boutique and ask if they can help authenticate it for you. Then, you can always re-sell it to her, if she is still interested, or, re-list it.
*you could ask your buyer to take it to a Chanel boutique and ask them to authenticate it for her, however, if it is NOT a Chanel, they most likely will confiscate it. Then, she has nothing, and you have nothing, and you'll need to pay her for misrepresenting the item. The tricky part here is, if she takes it, and it is indeed authentic, she still has the opportunity to tell you that it was confiscated (even if it's not).
which is why i suggest just taking a return on it, in the long run, it will be your least complicated solution. .