My DW and I also stumbled into this endeavor and used a real estate attorney. It was a little scary, but like MDM posted, I have no experience with the buyers not paying their monthly mortgage. For us, the buyer, for whatever reason, wanted to avoid banks and excessive closing costs. He voluntarily did this knowing the interest rate was twice what he would have gotten from a bank. (It's 6% for a 15 year loan.)
If you go through with it, don't forget about the mortgage interest as income for tax time. I just copied an excel file for amortization purposes and email my buyer the yearly update in January.
After my experience, I would actually do it again if the opportunity arose.