I hate to say it, but after you've sent him the $50 and he pulled the "I'm sorry, but I just sold it. I got something that is worth more in stock"-trick on you, trying to gain even more coins from you,
didn't get at least a little suspicious?

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1kvo38NsLZpTTGQxngpuFjJCtBnpX5fgF seems to be cloudbet.com deposit address (according to walletexplorer).
Another address that deposited into this is 1CvgpjjQoRb56qBVDWQYvnPsUwLUU5yHVn (Transactions here, here and here).
That address is mentioned in a scam accusation against AllGoodsBTC also involving GiftCards here.