--I would agree that this market has opportunities for seller finance as a record number of properties in the US are paid off. 

Nor have I been able to find an online application that I can fill out. 

I cashed out one little condo (rental property) at a time and paid the the taxes to do so.

The builder refuses to send them to you.They state they are 'restricted' or send the type of layout you see online, just boxes, nothing the designer can use.

I  bought my first flip for $120k (in hindsight, I probably paid $15k too Much).

He wnent ahead and signed a contract with her; within 5 days she was able to find a buyer who paid $75k more than she did.  

It's paid off, and is currently held by the trust she and my grandfather set up.

Goal would be to acquire 20 rentals by the age of retirement, with some already starting to be paid off.I am in the same boat as you, and I focus on safe appreciation all day (small cash flow for safety and then focused on markets on that appreciate). 

Or would this only work if they had it completely paid off? 

Gerard uses Appfolio to manage everything which is nice because of the online tools.