Part of the decline of newspapers, what part wasn't decimated by the excesses of private capital, came from Craigslist offering up a free marketplace for classified ads selling everything from socks to sex. In the past few weeks, in a cute bit of irony, I've been turning to the site looking for media jobs gradually being undermined by the service.
Fees, like captchas, are barriers to entry. When you charge for a service it discourages people from signing up and abusing (or even just using) the service as it changes the math of a break-even point for your scam. Newspapers offered that kind of barrier, though it was penetrated on occassion by the more ambituous, but Craigslist only attempts it for some real estate listings and ads from sex workers who, but for a wink and a nod, are pimping themselves to anyone that will look.
It is up to the community to point out the scams by flagging them and hectoring them with response posts warning their fellow users. For Craigslist the system makes sense, since there are thousands of listings posted every minute all across the world and it would be impossible to review them without grinding things to a halt, but in that darkness scams and shams proliferate.