Evaluating a rental property

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Hello everyone. I'm new to real estate investing. I might have stumbled on to my first investment. My brother-in-law's mom owns a hair salon with two apartment units on top. She owns the building free and clear. Her husband has cancer, so she's looking to sell the building to be able to retire, and spend more time with him. I'm interested in doing a seller financing deal with her. Right now I just need help with evaluating the property to determine a sale price. Currently the owner uses that salon for her business, and she rents the two apartments to relatives, who she charges far under market value for. When she sells the building, the salon will be empty and also be another revenue stream. If anyone has some advice for helping me determine the value of this property, I'd appreciate it. Thanks!