I think you are thinking of Form 1098, not 1099-INT. Form 1099-INT is issued by the payer of the interest, not by the person who receives the interest. The payer (such as bank that pays interest on an account) sends the 1099-INT to the person who receives the interest. Form 1098 is issued by a mortgage lender to the person who is making the mortgage payments, showing how much interest the payer paid to the lender. But neither Form 1098 nor Form 1099-INT is used for a seller-financed mortgage.
You do not issue a Form 1098. If you need to tell the buyer how much interest he paid you, do what SweetieJean suggested and send the buyer a letter with the information.
You do not need a 1099-INT to report the interest income that you received. Just enter the amount of interest in TurboTax as if you got a 1099-INT. All you have to enter is the payer's name, and the amount of interest in box 1. It doesn't matter that you didn't actually get a 1099-INT.
