You have a "seller-financed mortgage" if you sell your home and you accept a trust deed or other repayment promise as part of the payment. The interest the buyer pays you is seller-financed mortgage interest income.
Enter the amount of interest received into TurboTax as if you received a 1099-INT under the "Personal Income" tab. Enter the name of the party that the interest was received from in the "Received from" field, and the dollar amount of the interest in the "Box 1 - Interest Income" field.
This will correctly report the interest income on your Schedule B.