I would imagine it will look a lot like other online marketplaces where people will provide information to establish their trustworthiness (sometimes verifiable, sometimes not), escrow will be used (apparently already exists on the blockchain, though I'm not sure anyone has ever used it), traders are evaluated by those who trade with them for reputation purposes, etc. And ultimately some people will still get scammed, but that is hardly equivalent to a revolving door.

BTW, I don't see anything about a marketplace on the 2017 roadmap so we probably wont be seeing that on steemit.com in 2017 and quite possibly not 2018 either. For now it seems the focus of the steemit.com site is to be mostly for social conversation and fun, not commerce.