"How does Airbnb handle security deposits? Hosts can add a security deposit to their listing, as long as they add it before a reservation is booked.
If a host wants to make a claim on the security deposit, they must do so within 48 hours of the checkout date. If a claim is made, Airbnb will mediate and collect payment from the guest, as necessary.
Security deposits can't be handled off-site in cash, as off-site payments are a violation of our terms."
NOTE: I assume this prohibition on off-site payments only applies to accomodations. Obviously Airbnb cannot prohibit you from engaging in other business with the guest while they stay -- for instance if you are doing a garage sale, and guest wants to buy something you are selling, Airbnb cannot prohibit guest going to your garage sale and buying something. Or, if you rent a bike and guest wants to pay for that ---that is not payment for accomodations , so Airbnb can't prohibit you engaging in that business with the guest.