Be transparent about "honest wear." Small scratches on the body are usually fine, but any fungus, haze, or scratches on the glass (the "optics") must be disclosed immediately.
Check the serial number. "Gray market" units are genuine but were intended for sale in a different region, meaning the manufacturer may refuse to service them in your country. Where to Trade
Best for getting top dollar. Use protected payment methods like PayPal Goods & Services —never use "Friends & Family" for high-ticket electronics.