I just read a post by Seth Godin entitled "If Craigslist cost $1". I've used Craigslist and I agree with him that even a small charge for the service would likely clean it up a whole lot. At $1 per listing, most people with a legitimate need would still choose to use the service. But, for scammers and those with not so virtuous purposes, the small charge and thus the requirement for verifiable identification for the money exchange, would push them out of the game. Plus, the money coming in for using the service could go towards making the service better or making the owners richer or be used for philanthropic purposes or whatever.