On-chain, the solution is to have an oracle provide a third vote somehow. Off-chain, the solution is for the seller to report the buyer and its address to the authorities.
I can make a contract that relies on one single third party and in that case you are correct that there is some existing trust relationship. However, the protocols are designed such that the trust you need to have on the service provider is minimised with trusted computing systems and the contract's mechanism design.
Conversely, I can also write a contract that trusts a pool of oracles, or a prediction market (here the trust would be in the prediction market protocol and not on its operator), or a token curated registry where individuals are paid to give accurate information.