My wife asked me to sell this for her, so I'm guessing if you are looking at it, you know what it is, and why you should want it. This bag is barely used, and shows no signs of wear - but it is missing the shoulder strap and mirror.
They seem to be selling on average for $700 used, so I am listing it at $425 because of the missing pieces. Having said that, I'm sure you can find a suitable strap for practically nothing, and you don't need a mirror - you look great!
This hand bag lists for $1325 on the Balenciaga site.