Three years after the auction they can surrender their tax certificate and get a tax deed.

I sent a letter in the mail that gave her 75 day notice that her monthly lease would not be renewed (lease only required 60 days) and that she had to surrender the premises after 75 days.

Establish a residence, register to vote, obtain a driver's license (and make sure to surrender your CA driver's license), etc.  

She voluntarily surrendered her keys. 

The tenant shall at once surrender possessions of the premises to the landlord, and the landlord may without further notice remove all persons and all property from the premises.

If during the walk through, you determine everything is acceptable, I will turn over my keys and surrender the property to you.

Put that on the current owner...let him deal with the headaches, and surrender a problem/tenant free property to you at closing. 

In many jurisdictions, this would equate to surrendering possession of the unit.Ideally, you want to cross paths with them and have them sign a "return of possession" document.  

I contracted with them to handle screening; they have a no-refund policy once applications are singed and deposits are surrendered.

1) Cash for Keys and getting the tenant to acknowledge the surrender for keys in writing.2) Use a short period lease (3mo) which defaults to a m/m  continuance.