Yep, I’d want to put the car on a dyno and see just how many kwh come out of the battery. Seems to me that if one measures the current and voltage, it can be calculated. And rather than a hard price, the contract would be more like buying an existing home - you start off with a max price, but then bring down the price* after you get the home professionally inspected.
So, I’ll buy your piece of junk Tesla for 20k, but then take off $1,000 for each kwh that the Tesla battery is under, say, 75 kwh.
Perhaps I’ll start up a business doing that.
*depending on what the owner and buyer agree on...maybe don’t bring down the price