It's up to the seller to prove authenticity. You can do this by posting lots of clear photographs of the bag, its hardware (both on the bag and on the strap) and its tag, front and back. In addition, post photos of all the paper documentation, the extra tassels, etc., as well as a description of when you bought the bag, how often you've used it and whether you have treated it or not. If your bag is authentic, there is nothing for you to be concerned about.