Even if my tenant is late with the rent, I still check in on them and make sure they are well and the house is in good condition, and a clean environment for them to raise their children there.

Check out the BP events calendar as the next one should be added there for October.  

Go check the property out yourself.

Eventually someone who checked all surrounding properties and saw that mine is about 30-40k less have submitted an offer closer to the market price.

Thanks for the reality check. 

Not to get into too much detail, I had a transaction earlier this year where I represented the buy side, and that seller at the closing table had to write my buyer a check for $15k for not disclosing something that state law required them to. 

Did you not Check your property and do quarterly inspections ?

Check out the Ultimate Free Beginners Guide as well as other resources under the education tab above.

Over the last year and a half I’ve accumulated 45 units and haven’t done any official background check on any of the placed tenants.

You would probably want to check this with local commercial brokers and lenders though.