Hi A1pear,
welcome to the Apple (user-0to-user) Community.
Re: "AirPods Max authenticity: the serial numbers all appear to be invalid on apples serial number validation. Does this mean they are all fake or is there another explanation? Is there another way to prove authenticity?"
Am not sure where you are checking for serial number validation:
As far as I know, there are only a few ways to check if they are likely to be authentic:
Apple support's article: Identify your AirPods says:
AirPods Max. Model number: A2096 : Year introduced: 2020.
The model number is underneath the left cushion of AirPods Max.
Then the serial number: You can find the serial number of your AirPods in the same place as the model number;
Then there's the website where you can enter the serial number to check support coverage, and see what info is available .... or whether an error message is generated. Check Your Service and Support Coverage - Apple Support
All the best :-)