I dont usually find this sort of thing a problem because our company buy from large companies so the thought of getting proof is really not in our minds. I can see it inside the same stock inside other shops as well.
With some products that we sell, we have had issues with the brand holder in certain cases. They are basically checking to see where the product has came from, and are trying to find out what we are doing with the stock. When i then show them where the products were brought from and can show that we have invoices etc everything is ok. I am based in the UK and all our suppliers are from the EU and the UK so that makes it okay for us. However, if we were purchasing in amounts that were very tangible for us then I would really be looking for the paper work. However, if you are not dealing with a long established company then you do want paper work.
Using paperwork such as authenticity papers is not standard practice in my experience when you are dealing with meduim or small orders. An invoice from the company will act as proof. If you dont know who you are dealing with then proof becomes more important. On invoices it should say that the goods are authentic and free to be re-sold.