http://www.livebitcoinnews.com/meet-beaver-a-decentralized-anonymous-marketplace/ The paper describes an anonymous market by establishing a reputation system that is resistant to Sybil attacks in regards to vendor feedback and keeping customers private. The students evaluate the "shortcomings" of some other decentralized marketplaces such as OpenBazaar (OB) and the Silk Road 1.
See https://www.reddit.com/r/DarkNetMarkets/comments/4ju0b1/beaver_a_decentralized_anonymous_marketplace_with/
Also:
-_- Sybil attacks have nothing to do with user identities being private or not. They have to do with identities and reputation being easy to make and create fake feedback/reputation with. You don't even need zero-knowledge stuff for that. The way Beaver is preventing Sybil attacks is fees. (If anything, the zero-knowledge parts make Sybil attacks easier because you don't have to bother faking reasonable amounts and timings, you just dump n transactions of x coins each and no one can tell that they look super-fake.)