[tester756]

>How do you factor that?

I wouldn't because it makes no sense. Occurance of great deals aint relevant here, imo. Why would I care that customers do crazy stuff on 2nd hand market?

>I don't see how one would be able to identify and normalize all the required variables, e.g. median life expectancy, average performance across metrics per watt, average power usage, residual value, etc.

How about building system for similar price to Macbook and comparing their performance?

It, of course won't be ideal, fair, whatever, but ain't it what gamers do? They find PC configurations and check how games run on them.