Don't make the massive mistake eBay did, USE penny-start AUCTIONS for EVERYTHING, either that, or use Amazon's system of REVERSE DUTCH AUCTION. Wanna kill a marketplace and have a competitor beat you? Make all the listings fixed price. It creates clutter for buyers, it creates extra work for sellers (instead of just marking the GRADE of a common iPhone model which is a commodity, you have to repost ALL the information about the phone as if re-inventing the wheel), and it makes for a non-liquid marketplace. We've studied the history of eBay and Amazon's marketplace, as well as craigslist and Facebook market. The key is to create liquidity, and liquidity happens from prices being forced to a settling price. you should maybe do two or three versions, a local like craigs and FB market, an auction like ebay (but NOT fixed price offerings), and a reverse dutch auction for commodities (like Amazon's "used").