You can see reviews of the hosts, pictures of the rooms, and compare with other places. That certainly weeds out the psychos and serial killers way more than a random posting Craigslist.
You just really do need to ingrain into the community that a good reputation is essential, damnit, and don't buy from someone that doesn't have one. Then the sellers conform to that.
Once that happens, they move from buying or selling recipes for $0.99 to trading laptops, scam a few people, then vanish.