For the most part, I have used cash out refinances and home equity lines of credit to get the cash together to continue investing. 

If you make your monthly payment through a third party servicing company that then disburses to the seller it is possible they do report (some but not all do), but if you pay directly to the seller then that is unlikely.

I point out either it is open or part is not.

The short answer is:  your question is not scalable and requires buyer/borrowre qualifying knowledge and steps and thought not requuired for 1 per year depending on yiur states Safe Act's limits, its suggested that you use a 3rd party Licensed Mortgage loan Originator. 

We have a strong relationship with the seller and both parties are happy.

You can use other 3rd party custodian services.

After all, you two are the true principal parties in the deal. 

There is less risk when you don't have a mortgage, that part is true, but it is not as if there is no risk.

Written notice must be delivered by either party prior to the end of the month for notice to vacate in one months.

marketers and sales folks make fee's for putting two parties together