According to the website there is a web of trust to establish reputation and you can set up voting pools whose members are those you trust. So it is based on buyer feedback.
Most trades have a buyer, a seller, and a third party arbiter. When an agreement is made the bitcoin goes into something like escrow. Any two of the three parties together can release the money. In a normal trade, when the buyer receives the product he OKs the transaction and the bitcoin is released to the seller. If the product is not delivered or isn't what was promised the buyer can appeal to the arbiter. The arbiter settles the dispute. The arbiter can also release the funds if the buyer fails to OK the transaction
I don't think an arbiter is strictly required so if a big name company enters the market you can probably avoid paying an arbiter. Generally though, in a ebay like situation you'll probably need to pick a trust worthy arbiter and pay them accordingly.
https://blog.openbazaar.org/what-is-openbazaar/
https://gist.github.com/dionyziz/e3b296861175e0ebea4b