The point Trade was meant to be free would attract a lot.No terms and conditions to sign,no central authorities for restricting the trade seems good.Beating Ebay is far away from reality now as it is such a big giant in online shopping.But it can gain popularity among bitcoin community.
I am not familiar with Openbazaar as i have never tried it but at the same time i don't heard a lot about this project. For me it provides neither more nor less than a way obvious to trade "under the radar" and a commercial activity through it while avoiding taxs.