I'm a professional mediator just learning about bitcoins, and I have a specific question. I have been asked by someone if I am able to be the "third party" for a Bitcoin transaction.
In other words, Party A has X Bitcoins, he wants to sell them to party B, the X bitcoins are placed into a shared "escrow" wallet with the signatures of myself, party A, and party B. After the cash has exchanged hands (or not), any two of us (A, B, or me) can authorize the funds in the escrow wallet to be transmitted to its proper final destination.
How do I specifically do that? I mean specific instructions? I have accounts at Coinbase, Blockchain, my own Bitcoin-QT client on Windows, and others. What's my best approach to this?