From the project's page:
If you both agree to a price, the client creates a contract between you both with your digital signatures, and sends it to a third party called a moderator. These moderators are also folks on the OpenBazaar network—could be your neighbor or someone across the world—who the buyer and seller trust in case something goes wrong.
What if something goes wrong?
As we all know, things don’t always go smoothly. What if you’re buying a certain book from a seller, you pay the multisig, and they ship you the wrong one, or it was in poorer condition than advertised, or they don’t even send a product at all?
This is where the third party comes in. Remember that a multisig requires two of three people to agree in order to move the Bitcoin. They control the third key to the multisig, so the funds will not move until either the buyer and seller work out an arrangement themselves, or the third party agrees with either the buyer or seller on how to deal with the transaction and funds in multisig.
Seller: "I sent the laptop, here is a pic."
Buyer: "I got box with a brick inside, here is a pic."
Moderator:
ここには何もないようです