I am sure I have a ton more, as I've been in the same ~2000 sf house for 25 years that used to have 2 kids along with us 2 adults, and the house has always felt like it had more room than we needed.

I don't want to sign an exclusive contract with an agent.

In Chicago, there are management companies that focus exclusively on subsidized housing since it eliminates the rent collection issues, and this could be an interesting play in SB. 

After the purchase, I found out that one of the adult sons of the previous owner has been living there since his dad died.

She's an adult who signed a contract and, presumably, knows how to use the internet.

I don’t think it would be good to source potentially properties exclusively from something like this, for the reason you outlined.

You need someone who owns and deals with investors, ideally exclusively.

I want to build a private lender’s list to help with rehabs that I want to do exclusively. 

And I have an AMAZING lender that I work with exclusively.

We had our own strict limitations on who could rent our place that precluded large groups, and we rented almost exclusively to families.