No offense, I'm not calling you a liar. But there are some people out there that don't always tell the truth. If you've opened the box, you're going to have a hard time selling it as 'new'. People only have your word that it hasn't been used.
That's why most sellers on ebay or craigslist or wheverever, will take a pic of the item still in the box.
The guys are right, once its been bought from a vendor, its used goods and it needs to be priced accordingly. Unfortunately we don't live in a world where everyone is honest, so when we make a mistake, we end up paying for it. Good luck with the sale.