It might be ungenerous to call this a scam - however you should question why a long term guest does not want to commit to paying AirBnB, when there are so many policies in place that protect guests. Let alone protection from their credit card company if they pay that way. You should be concerned about a guest who is looking to make a partial payment.
You could tell the guest to book one month or even two weeks and if they like it they can then book the remainder and pay upfront.
Don't get taken in...