1)Quality is subjective, sales are not
2) How is this even measured?
The entire discussion is subjective. Quality and influence both. But as far as the discussion of sales goes, let's look at other mediums:
Highest grossing films of all time: Gone With the Wind, Avatar, Star Wars, Titanic, The Sound of Music, E.T., The Ten Commandments, Doctor Zhivago, Jaws, Snow White & the Seven Dwarves
Arguably all good movies (Avatar and Titanic are debatable) but are they the best? Where's the Hitchcock, Kubrick, Scorsese, etc? Where are the non-English language movies? Just because a given movie or album is more accessible to the public doesn't mean it's better.
Same with books. Best selling books (non-religious/political): A Tale of Two Cities, Lord of the Rings, The Little Prince, And Then There Were None, Dream of the Red Chamber, The Hobbit.
Does Agatha Christie really deserve to be in the running for GOAT author? Vanilla Ice went 7x platinum, Big Willie Style was 9x platinum. We gonna start talking about those as two of the best hip hop albums?