That's been the charm of Craigslist for awhile now: except for employment ads in two or three cities (and lately some erotic services), advertising has been free and easy. Which may be why the computer listings on Craigslist New York are now a wasteland of spam. This is an ominous sign for their business model, and perhaps another indicator that in the long run, for at least some important things, the "free" model on the Internet is not sustainable or perhaps even desirable.
If Craigslist had to charge for ads in order to deter spammers, that might erode the competitive advantage it has enjoyed over local news sites. The same would be true if Craiglist had to employ a lot of people to screen ads (and advertisers). But take a look, it's all spam, and I'm not sure why the same couldn't happen to other Craigslist categories and cities as well.