The steps that @Chris Luksha outlined are simple ways to reduce your risk by a huge factor that don't require professional abatement.  

The seller may not be ready to commit completely, but if you answer all of his or her questions, and you outline what he or she needs to do next, you have a much greater chance of closing the deal than if you leave the conversation with loose ends.Follow UpIf a seller is not quite ready to commit, but seems receptive to your information, offer to follow up within a day or two.

Repair cost: If you do take back the property for whatever reason, you might end up having to pay for repairs and maintenance, depending on how well the buyer took care of the property.The documents used in owner financing vary depending on the type of structure used, but in most cases, there are two separate documents:The note, which outlines how much is to be repaid and the terms of the repayment. 

In this case, since it sounds like you are okay with them moving and if there is no time outlined for a notice, you may consider giving them a notice to terminate at the expiration of your lease allows. 

I find that when I take the time to research what has sold in the area in the last 6 months of similar properties and hand the comps to the appraiser, and I outline the improvements I've made to the appraiser, I tend to get the number I am asking.

Then as a condition of closing, you outline what you believe he told you and include the need for a copy of the signed lease agreement.

That being said the broad principles as outlined in much of the Biggerpockets community of: (saving money and not letting lifestyle creep prevent you from meeting your goals, buy right by finding under market deals or "value", do actual analysis on your investment, go into every investment with a plan that includes how it will be managed, rehabed (if needed), financed, and with an exit strategy in place, and continue to have time in the market (in our world the real estate market), are principles that can be applied for any investor looking to grow their net worth and income via real estate.

Agree with @Eric SchultzIf you want to be passive, the above outlined plan will do so but it does take some upfront time devoted to education. 

@Steve K. did an exceptional job of outlining key first steps to take.

Note too @Dan Heuschele (who had excellent thoughts), it is possible to do a part gift/part sale, which could alleviate some of the concerns people have outlined.