I don't have enough info to make specific suggestions, but first you need to have good communications about the financing. I always assessed rates after underwriting the risk, but I understand most can't or won't do it. Working with a third party broker will also help.
Also, compare your rate to the going closing costs of conventional loans. Maybe 2 points, underwriting fees, funding fees, survey, appraisal, filing fees for a 50 page closing, escrow fees, servicing fees and the list goes on. Now, compare the higher rate of your note rate. Closing costs are front end loads, paid up front or financed, your interest is paid over the term the "money" is used. If they re-fi your note in three years, say two years early, they save money.
Sell the benefits of seller financing! You don't have LTV rules initially, so they can get in cheaper and build equity, make improvements and then use the future value of the property to qualify for practically the same dollars!
One other thing, even if you are not under the new rules, you should proceed as if you were, IMO, make it fair, justify your position and why you made the loan. Good luck