With the basic understanding that it might be best to walk away if the car is significantly worse than what you thought when you clicked "buy it now"
Here is a checklist I found here on the forums and have added to.
1 2x key fobs
2 Supercharging enabled
3 Autopilot enabled NA
4 All tires match and clear of issues (unless photos showed otherwise)
5 Charging kit with all adapters (including J1722, NEMA 5-15, NEMA 14-50)
6 All lights work
7 All windows work
8 Wipers work
9 Speakers work
10 Parcel shelf present
11 Rear window glass pitting
12 Sunroof opens/closes
13 Power lift gate function
14 Frunk function
15 Steering wheel controls
16 Both seats function
17 All seat heaters
18 Wiper blade condition
19 Previous user settings/details wiped
20 Miles match what is advertised
21 No paint/panel issues beyond what was in photos
22 No rim issues beyond what was in photos
23 Front and rear floor mats
24 VIN matches paper work
25 Check mirrors work and fold in
26 Check charging port door opens (and closes / latches)
27 Rear seats fold up and down
28 Check rear lights for condensation
29 Front license plate holder
30 Check windscreen for chips
31 Take photos, notes and request due bill in writing for any damage noted
32 Request service history (they will likely decline due to privacy issues)
33 Check LTE connection
34 Connect phone to check Bluetooth
35 Check range on battery in % and miles
Calculate rated range at 100%
Sudden Loss Of Range With 2019.16.x Software
36 Run the HVAC
37 Check jack points and battery pack for damage
38 Check battery / car can get to full power acceleration
39 Rear defrost, if possible
If an earlier D model check the front suspension for wear / cracks / interference at full lock
Cracked forelink?
Obviously not all of these can be checked at all places and situations. I customized the list with expected values so I had them all close at hand when I was working through the list. For example I noted the mileage and marks on the car to check.
As discussed, these are used cars and "as is" to a large degree, the goal is to assess the car thoroughly and understand its actual condition before you sign the paperwork accepting the car. If things that they won't fix are bad enough, then you may need to walk to avoid being seriously upside down on the purchase price to real value.