Best ways to get an agreement from landlord to purchase SFR?

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The landlord that I currently rent from might be interested in selling soon. Single family residence in a booming college town. How do I go upon securing a possible agreement before they list? I have a great relationship with them (done exterior painting and small things for rent deductions) and I believe they would give me a good price. I would most likely rehab rent refi hold if that is a likely scenario and makes sense.