Computing ROI when offering seller financing

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I am terrible at understanding ROI. In my bottom-line thinking, there's either profit or there is not. Assuming I'm using my own funds, I understnd there's ROI when I buy and sell, but how do I figure ROI when I add seller finance to the deal?

Here's one I did a few weeks ago:

Purchase price was $5K. There were costs so let's say I'm now all in at $6K. For simplicity let's say there was no hold time, since I sold it a few days after purchase. Sale price was $12K. Terms were 1K down, 6% interest for 11 months, 11 payments of $1030.25. Total time to pay off is 12 months.

What's the formula for ROI on the $6K outlay?