Airbnb has done a great job so far, but I think there needs to be some more effort put into validating the renters information. There are several ways to do this via online identities, credit cards, etc. The other option I would definitely add is a specialized rental insurance of some kind. An additional $10/night across all rentals should help pay for situations like this along with the basic broken item situations that I know you deal with. Let the homeowners decide if they want to charge/pay that fee. Then it is in their hands if they choose not to.