Could be legit but why, when you offered $300, would he drop it to $200? The box isn't worth $100!
Maybe he doesn't understand negotiating.
Seller: I'm asking $500.
Buyer: Would you take $450?
Seller: Um....I don't think so. Give me $200 and it's yours.
Like I said, it could be legit but I'm suspicious. My son, who is a MASTER negotiator, said he'd walk away from that deal. He'd be too worried it was stolen. (He made that mistake once with an ATV. Great deal , nice guy, even got a bill of sale. It was stolen. My son's mistake was that he never checked the guy's ID. The name on the BOS was fake. I pity the poor guy if my son ever sees him again. ).