Here is what we do. I will use a 100k house for easy math
You have a mortgage on a home. Place it into an inter vivos land trust. This protects you from Due on Sale, is easier to get them out if they do not pay, and will give the buyer the security of knowing that you cannot evict them unless they do not pay. (you will need a duly assigned fiduciary for this to work)
Charge a 5% to 15% investors fee for doing the financing.I will use 10% for the example (roll the fee into the loan) Charge whatever interest rate you like then amortize it over however long you want. Charge whatever down payment you want.
Breaks down like this
100,000*1.10(investor fee)=110,000
110,000-15000 down payment=95,000
Now roll closing costs into the loan ill use 5k for easy math
95000+5k=100000.00
So lets say you bought it for 80k, sold it for 100k+10%investors fee and then financed 100k.
Set your interest rate by how much you want their payment to be above your current mortgage payment.