You clearly seem to think you're in the right here. You might want to read up on more law before doing something so sketchy, next time. The legal tender point is irrelevant; you are in possession of someone else's property, regardless of whether it's an "official currency" or not, and you demonstrated bad faith by selling it all immediately. Your point about it acquiring value the moment you sold it makes no sense. If it didn't have value before you sold it, nobody would have bought it from you. You might also want to read things like http://www.austinchronicle.com/columns/2005-12-02/315676/. Whether it's an intangible good or not is also irrelevant. You did not _take_ someone's property, but you did not make a reasonable effort to return it to them, and instead posted on facebook (of which there is documentation) about "free money" and started preaching about how the victim was in the wrong. Definitely not good faith, and it'll look bad when the law gets involved.

I could accidentally send you my collection of used women's panties (hypothetically ) and if you kept it, you'd be liable. No, used panties are not legal tender. No, they have no market value. But you still took my stuff, and made no effort to return it to me. Sure, you wouldn't be liable for much, but you would definitely be in the wrong.

Over $4000? You should be worried.

Edit: Also, what's with posting his picture and name? Who cares if you know who he is? Are you trying to intimidate him by suggesting you know who he is?