Sales matter. Music quality is subjective; therefore, the more people who purchase a piece of music means the product is better than the lesser purchased product.
This doesn't make sense.
In this case, Eminem has the best music out.
And...we know he doesn't.
But the fact that we can argue about "how good" his music is, is the point.
There isn't supposed to be an objective answer to that. By using "sales" you're just trying to use the band wagon instead of having to actually reason out why you support an artist's view on the artform.
The sales argument is for lazy people who are don't like formulating nuanced arguments about taste.