approaching neighbor about off market sale

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Hi.  I learned that a neighbor is interested in selling her property.  With zero negotiating experience, can anyone provide pointers on how to approach her?  (I don't know her myself, and I do plan to speak to her in person.)   My biggest hang up is that I have no idea what the valuation would be to eventually start a conversation about selling price.  The property doesn't even show up on Zillow, so I don't even know the bedroom count etc.  Her sister lives with her and plans to move out of state, so she is interested in selling apparently.  

Any advice is appreciated!