It's maybe time for Craigslist to grow up and start charging fees, at least for big-ticket items. Their anti-profit approach to business is annoying because it only moves the profits from where it could go (craigslist) to another place (all the people selling things). If you think greed is evil, shifting it around in a zero-sum way isn't doing any good.

Charging is not the holy grail of spam detection. There are services that should remain always free. CL can't (or shouldn't) charge for posting in categories where the user is not going to perform any economic activity. Charging an amount (no matter how small) in categories such as Personals or Missed Connections (besides not making sense at all) will decrease the amount of legit postings, ruining the quantity and quality of the service. By turning into a pay-per-post site CL could become something like ebay (a pure commercial "I sell you my collection of Russian dolls") + job postings site. That is, trust me, out of question.

Those non-commercial categories are the ones which are receiving far more fake postings, so the problem on the commercial categories could be considered closed. In non-commercial world, there are several measures being taken care of, such backend data analysis (the most obvious choice) and phone authentication for posting on a small set of categories (such Erotic Services).