I don't think your calculation of $600/year in interest is correct to represent 5% APR. If you finance $360,000 and show only $600 as interest income, then you will have shown an APR of 0.17%. ($600/360,000 x 100). This will not meet the minimum. Remember interest rate is on an annual basis, not on a total basis. If the seller were to put $360,000 into a 1.5% CD he's getting $5,400 a year. He's getting the shaft with $600/year in interest.
Also with the nature of installment sales (which is what seller financing is) the seller is going to have to take the agreed upon sales amount divided by his cost to come up with a gross profit percentage. This percentage will be multiplied by the installment payments to come up with the profit he's going to have to report each year. So the monthly payments will have three elements for the seller: 1. Principal reduction 2. Gain (profit %) 3. Interest income element.
Even if the seller sells the property at his cost, that only takes out the gain and you still have to show the minimum interest income.