They must charge fees, or they will get spammed. You know the drill.
From this blog post:
OpenBazaar will always be open source and MIT licensed. As a global software community, weve intentionally created OpenBazaar so that there are no fees required to use the network, and there is no central authority controlling trade, taking a cut, or monitoring data.
http://avc.com/2015/06/future-friday-openbazaar-and-ob1/I'm no expert on the technology so I'm not 100% sure how it works but it might be best charging minimal fees and having them distributed to the people who run/host the client in a similar way to how bitcoin mining works. Does anyone know how the escrow function will work? That could be the trickiest part or weak link in the system.