I recently bought some bitcoins, via a middle man, who I can trust (hopefully). He has guaranteed that he will pay me the BTC, if the seller does not transfer them. Is there a way that I can prove to him that the seller has not payed, if that happens? Let's say that the bitcoin adress was 15LiGv6kVev6cAWvKRNuo2pNLEeRQ631cn and that I sent him this beforehand, to send to the seller.
I still think I will get the BTC, I'm just curious on the scenario!
All transactions are in the block chain and you can view them in block explorer or via the command line. You only need to guarantee that his address, or at least your address, are the actual ones, because anyone can point to a transaction and say, "that was mine." You might need to show the guarantor that your address is your own, perhaps with a miniscule transfer, unless you are both physically present and view your wallet's addresses or something.