Three-year-old Airbnb is a sort of eBay for places to stay: a marketplace for spare bedrooms and unused vacation homes in 190 countries that matches travelers with hosts and handles the financial transaction in return for a 3% cut. It's gained a reputation as a more economical, personal alternative to chain hotels, and has inspired countless glowing testimonials. But in July, the site had a p.r. crisis when a host blogged about a nightmarish experience with a destructive, pilfering guest. After a somewhat shaky response, Airbnb responded by instituting a $50,000 insurance plan — and all evidence still suggests that deals gone bad are one in a million.