I'm not familiar with the architecture of openbazaar, but whereas with centralized services like amazon and ebay only your credit card and the marketplace have access to your full history (and people tend to trust these entities, begrudgingly or not), in a p2p model, the marketplace is everyone in the network. While for most people its reasonable to trust a reputable organization for their legal transactions, very few people would be comfortable letting any scriptkiddie with an hour of time analyze the public network activity and get access to their entire purchase history. In order to avoid this drawback of a p2p architecture, additional privacy guards must be used.