My seller is a 77 year old landlord looking to retire I promised him that I was going to get him the cash that he was looking for.

I can promise you, if you find a good deal and fund it there will be plenty of contractors who will want to do business with you. 

And if you exit early, and get her loan paid off, well you've done what you promised!

I dont know because from what you wrote I know very little about this property and you had provided a very large value range 373k to 500's thats huge discrepancy.The other question you asked was a legal question regarding  protection between the tenant and their work they promised as "consideration," or value for the rent.

Here in Ohio there's a few smaller towns (<100k population) that show some promise from my estimates.  

Can anyone tell me what they look for in a promising property ad, that will send either red flags or green?

I promise you, they will be the most demanding investors you have. 

Most people see the ad and know it is a fake (rooms for rent are usually more than that.) 

Are you wanting him to make up a fake invoice and you deduct it as if you had paid it.  

If you can you should post in market place section under "Real Estate Jobs"  as I think about it that may find a realtor for you, but you'll want to incentivise (hmm, not sure if that's a word) the arrangement by promising to pass on to the realtor deals you find unworkable.