I too used cozy and all of my properties/rent collection was automatically transferred over to apartments.com.

Two years ago just giving it your best shot on theming meant automatic success. 

Does the contract automatically become null and void or does it give you, the seller, the Right to terminate?

Automatic.

Her complete mortgage is paid each month from the AirBnB income, and she rarely even sees her guests because they have automatic checkin to the suite and no access to rest of the house.

They seem to still offer the service you are looking for Pay Rent Online - Automatic Rent Collection for Landlords | Apartments.com

My favorite thing about it is the automatic late fee you can setup. 

Once you have a written Policy, provide it with your application, and have it available at any time you are showing or meeting with prospective tenants and automatically include it with your Rental Agreement package.

The best way to make sure you're paid is to use one the many websites that will collect the rent for you,...automatically. 

It turned out that 70% occupancy was required to stay in the black.With a fixed rate mortgage, you're trapped into a payment scheme, while the ARM will automatically 'self-adjust' as you pay down on the principle.