Try out my friend Google - "installment sale tax treatment"

Did the person sells it's shares in the C Corp or did the C Corp sells it's assets? 

Likely still an installment sale, but on which return? 1120? or 1040?

the taxpayer sold the shares of the C Corp....Is it possible to make an installment sale when we do not know the total Sales price now....? taxpayer would be receiving payment every year for the next 3 years based on profit performance...

You do know a sales price, so yes. It is possible that the total sales price may decline in the future and the 6252 may have to be adjusted. Do some serious reading. https://www.irs.gov/publications/p537

Perfect...Thanks!