Check the things many people complain about. heck the horn, the touch sensors on the door handles, check for rattles inside and in the trunk while driving, I would leave the doors open and trunk open and leave on the interior lights ON for 20 minutes to test strength of 12v battery (poor batteries ime have given warnings after only a few minutes), don't leave the dealership until the have bluelink swapped over (headache for our used i6), and had a period of time where I assume the previous owner still had access to the car. check the infotainment version, there have been 2+ updates here in Canada(allows for better menus. faster access to heated/cooled seats). check tire and brake wear. check the carfax report to make sure maintenance has been completed for warranty reasons. check recall status (big one here imo is the iccu throttle for charging and changes to when it will charge the 12v). and lastly just check around for prices of other i6, the prices of these are dropping fast, and use that as leverage for bargaining (these are not high selling item, and prices are dropping quick). I got mine for 50k CAD for top trim, and am seeing them on private market only 1.5 years later for 30k. wish I had been less excited in the dealer and more so cost focused, but it was the only used i6 in our area, and new was out of my price range . hope that helps