It's a good warning to be leery of taking out lines of credit on your own residence for REI.

Heed the warning bells in your head. 

Passive-aggressive will be taking their anger out due to having to fix this problem by not warning you of a steady drip in the water heater until it explodes and ruins the flooring in the adjoining rooms.  

Is that a warning sign?

In addition you stay on top of a brand new tenant in those most important first 3 months of the lease, keep them in line, watch out for any warning signs, squash any issues like a bug and be ready to evict at one false move made by them.I don't  put a lot of faith in 'collectable' versus 'uncollectable' tentants as almost everyone is uncollectable to one degree or another.

To set-up for payment processing on your own and pass PCI and/or ACH requirements can be expensive (development, audit and assurance) and, as @Jimmy Moncrief warned, the liability is almost certain to outweigh the benefit of brand control.For a small operation, I would recommend you find a storefront hosting provider or payment gateway and simply throw the session to it from your website.

We did not renew because she disturbed other tenants even after repeated warnings.2. 

Go there in person and say where’s my rentc.Call them up and warn them that they better have the money or else?

They care about you, so they want to warn you of these dangers and protect you from the hungry lions they see everywhere except within the "safe & stable" bounds of a well paying 9 to 5 job.You can't fault them for caring.  

Thank you for the warning.