What should I be asking the seller when I'm buying and Audi A6 2.0 L TDI with 80K miles as part of my due diligence ??

The first question will be why are you selling it and the second question will be can I see your maintenance records. I love Audi but they are expensive when it comes to some repairs. Therefore if it is a dealer you are buying from ask about the warranty, if a private seller then you can purchase a warranty. I would go to a trusted mechanic and ask him to give you the names of a few warranty companies that offer good service. Also I would like to get a second opinion on the car from that same mechanic if any doubts linger about what you are buying. Also you may purchase a vehicle histiry report from Auto Check or a company like that. They will give you a report of any accidents etc. on the car. Hope this helps.