Thanks for this. But then who acts as the third party?
Someone who both buyer and seller trust. If you need escrow, make sure the escrow agent has good feedback from other users and make sure to not trust them with more bitcoins than the reputation is worth to the agent. (I mean, if I make 50BTC each month being escrow agent, then you can trust me with 1BTC escrow, but not with 500BTC)