We all want to get a bargain and many times we trust the seller who writes "Authentic" or "Genuine" in his/her listing title. Unfortunately not always it is true. I have had a few experiences where I trusted what I read in the listing and when the item arrived it was obviously a fake.
I normally ask the seller to confirm the authenticity of the item as this will put him/her on the spot. Then I might ask to see more photos: especially of the finishing but more importantly of the labels. Very often the writing on the brand label is slightly different from the real one and most importantly the contents and washing label in the fakes will just say i.e. 100% silk and made in China or similar while the authentic item will most often have been made in Europe (mostly Italy or France) and very often name the specific factory used.
If you are not sure when you receive an item query the seller and most often you will be offered and return and refund.