The trust factor is the difference: you can't fake cool, and you can't fake integrity. Whereas eBay (these days, anyway) lets scammers "blow-up" auctions in its pursuit of listing fees (http://www.nickchandler.com/blog/ebay/nigerians-provide-free-shill-bidding-on-ebay.html), Craigslist cares about people abusing its listings -- check out Craig Newmark himself replying to a blogger's complaint about the apartment listings (http://www.mandellonline.com/?p=81#comments).