The interest gets reported on a Schedule B.
Go to Your 2016 Income Summary.
Go to Interest Income.
Select Start or Update by Interest from Seller-Financed Loans.
To see if it was reported,
Go to My Account.
Go to Tools.
Go to View Tax Summary.
Go to Preview my 1040.
Your interest on the loan should be reported along with all other interest on line 8 of Form 1040.
