I can't speak for Craigslist, but in my area Gumtree is big, and I know from first-hand experience that they "handle it" by waiting for the crime to occur and be reported to police, then they give police the list of all IP addresses that viewed a listing. Having stared down the pointy end of a knife right outside my own home, I feel there's an opportunity to build a better system that keeps people honest and discourages thieves.