It looks like it uses multisig as a form of escrow [1], but the third party isn't in any way affiliated with openbazaar, just whoever the vendor and customer can agree to trust. It looks like users have to find a third party on their own, out-of-band [2]. I can see this working for small vendors, but I don't see how it would work with large vendors without the emergence of 1–3 large escrow services, which would diminish some of the benefits of decentralization.
This kind of multisig-based escrow will never be able to compete with Ebay. On Ebay, customers don't realize that they're paying Ebay to be a moderator because it's included in the price tag. On OpenBazaar they will be forced to pay attention to the escrow fee. I also fear that it will be impossible for escrow services to develop the necessary level of trust without being forced on vendors by the marketplace and still compete with the customer-favoring escrow that Ebay has.
It competes much better with darknet marketplaces, since it eliminates the possibility of sudden cash-outs or FBI raids on the scales that other markets have seen. They should accept that that's where they'll be successful and integrate some kind of anonymizing layer, probably tor, maybe i2p.
[1] https://blog.openbazaar.org/what-is-openbazaar/
[2] https://openbazaar.zendesk.com/hc/en-us/articles/207548366