The thread title is misleading.
Sales don't reflect MC skills. Sales reflect an ability to make hits.
There's hundreds of rappers out there that can spit fire. But there's really only 5-10 people at any given moment that can make hits. I'm not talking about free styling over somebody else's beat. I'm talking about making a song that program directors at radio stations play 20X a day because it's the hottest song in the country.
Take WU Tang for example. They can all spit flames, but it's really only Meth, Rae & Ghost that ever made hits (correct me if I'm wrong).