Subject-to financing monthly term question - newcomer

Hello Community,

My Day #1 on BP, greetings. I wish to beginning pursuing subject-to/seller financing investment properties in my native SoCal region. Can any experts explain the outcome should a investor initially agreed to monthly term payments to the original owner but then decides to sell the property (provided no upfront payment/payoff)? 

I am also seeking affordable, creditable, and effective tutelage if you have resources. Thanks!