When you buy a brand name item from an authorized distributor, they should have no trouble in providing you a copy of documents to prove the product's authenticity - so request for one at all times!
A few sellers will bring the item to a shop and have them authenticated, for example some sellers bring their Gucci bags to the nearest Gucci store in their area for authentication. Many will have this done especially for high end products.
You can also use the official receipt to prove the authenticity of your product, like there was a sale in one of the Nine West boutiques in your neighborhood and you got a purse there, you can add the O.R. (just black out the price) to your listing/photos as proof.
Stay away from any Chinese supplier selling brand name items, you will never get anything authentic through that venue.
If it's a used item then list it as used, a few will use the term "pre-loved". But if it's not then you shouldn't list it as so.
Hope this helps. Please let us know if we can help you with anything else :)