Seller financing

[Ali Chami]

Hello all, I was looking for some advice or input on a RE transaction. I began buying SFRs and rehabbing them with the intent to hold them for a return (also, I always keep my options open). So far, so good. I have 5 under my belt and the renters are lining up, thankfully. On average, my final cost after acquisition and rehab is just under 20K per property (all appraise at 60 to 70K). I am collecting 800 to 900 monthly rent per property. Taxes and insurance range from 2,500 to 3,500 annually on each. The return is very good (25 to 30% ROI on each).
On one property, a prospective tenant wants to purchase it. We agreed on 55K purchase price, with 10% down. When I ran the numbers, however, on a 30 year note it does not look very good for me if I hold the note. Of course, there is the option of selling the note later on, but there is no guarantee. Renting it out would be a much better return for me.
Then I re-ran the numbers based on a 15 year note at 9%. I proposed this to the tenant and he was quite pleased (even at a 15 year amortization, his PITI would still be lower than the 850 rent). The monthly payment on the note is a little less than what I would net if I rented it, but not by much.
But is my return now 9% (the finance rate)? If not, how do I calculate the ROI? If this works out, I would be in a position where I probably would not want to sell the note. Any insight or advice? What are the perils of seller financing? Part of the requirement for financing (assuming it is allowable) is that I draw the payment directly from the buyer's bank account every month (spoke with the branch manager at my bank and she told me that we can do this at no cost).

Thanks,

Ali