Does anyone have an opinion on if it would be worthwhile to try contacting the Greek police in regards to the above threat?
You'll hardily achieve anything meaningful.
You'd have perhaps more chances by attempting to talk with exchanges, since you probably know the owners of many of them, asking for an eventual block of this individual if he ever tries to cash out the money he owns you. But I'm not sure exchanges would accept to do this, as it could backfire on them.
I don't know what you can do. Fortunately it's not a huge amount, it won't hurt you that much.
50 dollars just isn't worth the stress mate.
On the other hand, prospective customers may be put off by knowing that in the event of a dispute, their personal information is not necessarily safe..
This is true.