Are you serious? He sold them before the conversation started, so his intentions were very clear. You think a nice tone would have changed this?
Good to know how to approach scammers for the future though: "Could you please give me my money back?

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Yeah, it could have changed things. Dude upon waking up and finding 500+ bitcoins may have gotton excited and let his greed get the best of him. As easy as it was to sell, its just as easy to buy back. But we'll never know because both parties wanted to be a tough guy.
How to approach someone who has you by the balls? Hint: Not forcefully.