Personally haven't bought anything off of Chrono24, but have purchased from other sellers and had items shipped to me. Every time the watch was shipped, it was with overnight shipping, well packed in multiple layers of bubble wrap and foam. Usually came in a box or protective case and was exactly what was shown in the pictures.
Have you contacted the seller with pictures of how the box arrived on your doorstep? Once you unpacked it? Can you pull up the original ad with a pic of where the scratch is showing it wasn't there before? Was this a brand new, unworn watch? (you mentioned stickers so was it advertised like that?)
Due you diligence, contact the seller (or Chrono24) explain how it showed up and go from there. Not sure on Chrono24 policy but would think things like this are covered.
Best of luck!