So you are asking how to recognize for sure someone is (not) cheater ?
No way to be sure. Some risk is part of every business.
Not only your peer may lie he is authorized reseller while he isn't. He may claim name of true authorized partner while he have nothing to do with claimed company. I have no personal experience with such kind of true cheating and I consider it rather rare.
IMHO, that primary question is - can I trust *this* particular seller ? Will he provide support to me ? Will he replace damaged box if necessary ? Or will just kick me "such model is obsolete/no longer available, you need to buy new box from me" ?
Forget free lunches. There's nothing like it. I have not so good experience with true authorized partners. Remember - business partner is not your friend. He is interested to sell you, not to help you.
Your primary interest is to buy from trusted partner. According my experience, authorized doesn't mean trusted. Moreover, there's no direct relationship between current box price and future willingness of seller.