I have $500k in cash to invest as well as a credit score between 750-800. 

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

If the new buyer defaults on payments or otherwise breaches the terms of the mortgage, it could impact your credit and financial situation.Market Demand: Consider the market demand and potential buyer pool for properties sold under subject-to arrangements.

I'm trying to refinance so that I can free up my funds from my credit line that I bought the property with, so that I can repeat the whole process with another property.

I allowed the original owner to stay in the house until payment was completed and was credited the $300 every month.

I put a link from Rentredi if anyone is interested to look.My screening criteria is 3x monthly income, Credit Score >600, no Felonies, and I have really be on the fence pets or no pets depending on size etc I need help with the tenant selection1.

Not to say banks do not sell loans to free up credit lines but in most cases less hassles and headaches.

Has anyone figured out which credit unions are offering this product?? 

No the one year occupancy is not just ignored for the seller.No idea what Mom's income, credit and assets are so cannot tell you if she helps your profile or not.

Maintain your credit and bank balances.