Your LEASE should address what is considered valid written notice. 

Kemp II, there are a lot of valid comments from everyone.

Out of morbid curiosity, if he was asked why he isn't paying you back, does he have an actual reason even if it's not valid?  

Pay close attention to warranties. 

Is that valid?

There is a 1st year bumper to bumper warranty,  2nd year plumbing and elec, and a 10 year structural. 

Our standards include that income must be 3X the rent, credit score above 650, clean background check, 1-2 past landlord references (the more the better), 3-6 months pay stubs (because of COVID, we requested more months), valid social security and ID and so on...

I should have asked the Seller to provide a Home Warranty since I knew the HVAC was older and I wasn't planning on replacing that originally.

You should first talk to a title company to see if you can get title insurance on a tax deed, in the event that whoever buys it wants a warranty deed rather than a QCD. 

Have you seen bank statements that show the current owner's income to validate the rent income?