I do not believe it would be illegal, but the IRS might object to the characterization of the loan as principal-only payments. They might insist that the sales price was less than the price agreed and that interest was built into the purchase price. If the sale price was well-documented as being FMV, it would seem like a difficult case for them. I could see why the IRS would object in some cases though. For example, imagine a owner-occupant selling a home with significant appreciation and owner financing. If the house was purchased for $500k and was now worth $900k. A married joint owner-occupant might want to characterize the sale as a sale for $1m financed at 0% interest. All of the gain would be excludible. In contrast, if the sale was $900k with X% interest such that the payments were the same, the portion of each payment that represents interest would be includible in income. A CPA might have more thoughts.