The beef predates a time when sales were even relevant to the two, since it started prior to the release of both Cloudy Eyed/Mitch Bade and Genuine Liqua Hits. Tech's "baby momma" went over to Vell and got in his ear which led to Vell going at Tech on Genuine Liqua Hits. People then told Tech 'yo, you need to check this album out, he's going at you' so to speak. A few "..."'s later, you got shit like He Wanna Be Paid and Cracker and such.
EDIT: I'm not positive, but I got the feeling this was covered in that big interview RMiller did with Tech. I'm thinking this because Miller took questions from people on here, and thus asked interesting questions that didn't lead to the same five "rehearsed" answers, haha.