How I deciphered the Airbnb message on security deposits:
If you have set a security deposit amount, Airbnb will authorize that amount from the guest's card 48 hours before they check in, just to check if they even have that much money on their account - for any reservation made.
If you don't have the security deposit set, Airbnb will decide the amount on their own, calculating % of the reservation amount.
In case Airbnb can't authorize the specified amount from guest's card, they will automatically cancel the reservation up to 36 hours from check-in (because it's a hassle to extort the money from guests later if damage claim is made after the stay) = last minute cancellations.
So, right now I have the security deposit set to 300 €, because Airbnb was not holding it from the guests before and this was a safety net if anything happened, I could say the deposit was set that high from the start.
That means that the guest will pay XY amount for their stay, and 48 hours before check-in, Airbnb will try to take the extra 300€ from their account. If that fails, Airbnb cancels the reservation.
I might (will have to) lower that amount now, because I'm afraid I will get more last-minute cancellations because guests won't have enough funds on their cards to cover the security deposit that high. But then, how low should I go?
Now, questions...
How will Airbnb calculate the security deposit amount on their own? What % of the reservation amount will be added as a deposit? 10%? 50%? I would be happy with, let's say, price for one night stay, but then again, our prices are quite high - someone who rents for 15€/night will want a higher security deposit, and will have to set their own amount.
How long will Airbnb hold the security deposit? It should be at least up to 14 days after check-out - the whole period when the claim is possible to be filed.
That's my input how I understand it.
HomeAway has a different approach: The guest pays for the stay + security deposit at once. After their stay, hosts get a message asking if you would like to return the security deposit, or it will be refunded automatically after 7 or 14 days. However, the commission fee/payment processing fee also calculates in the security deposit amount, so when you refund it to the guest you also get the fee refund.