My two cents. Don't listen to me, but the price is not the biggest issue here, in my opinion.
You can circle back to price later, but first, talk more to him about his intentions:
"You want $1m for this duplex huh, how did you get to that number?"
* "What about any damage (give examples)?"
* "What is the age of the roof?"
* "Age of HVAC?"
* "Age of water heater?"
* "Age of appliances?"
* "Any foundation problems?"
* etc. You get the idea...
"So, you want to 1031 to buy another deal. Do you have the deal lined up now?"
"How much money do you need for the deal?"
"If you had that amount now, would you still want to 1031 the duplex?"
"I'm trying to understand intentions. What's important to you?"
"If you really don't know the house's value, considering any maintenance and/or damage, maybe you can get an inspection. Or at least consult a real estate agent. Once I know your true underlying reason to sell, with your understanding of the value, we can work out a custom-tailored deal. I'm excited! I know if we work together we can figure everything out!"
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Once you get to the bottom of everything then you will know what to do. Who knows, maybe you could get a loan for the amount he needs for the downpayment (if it's not $1M), then do owner financing for the remainder over time. Don't be too hungry to make a deal. Be willing to walk away. Explain your terms and if he can't get what he thinks he can, he may circle back to you later.