Selling Small Single Occupancy Commercial Bldng TX - Questions

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FIRST - My husband and I have had a couple of residential (single family) rental properties in the past but have never had any commercial real estate. We purchased this small commercial building in far North Texas appx 8 years ago from a friend as an investment. At that time we updated the electrical, heating, and in Dec 2016 put on new roof. The building is 100+ years old, just over 1,000 sq ft, located in downtown of small town and is leased to same tenant for 3 years. We are entertaining selling and the tenant is interested in buying. We have not listed with a realtor and will be finding a real estate attorney to draw up papers. We have pulled a selling price 'out of the clouds'. We have looked for an appraiser to provide solely market value to see if we are in line but the appraisers located in the area do not seem interested in this very small project. If the Tenant is unable to obtain financing through a bank, we are thinking of doing an owner finance and if we do, how is an appropriate interest rate determined.