All I can say is if I'd used those methods in my degree I'd have failed. To me, a sales tracker tracks sales. It doesn't guess, look at past trends, or anything else. If, for example, the entire population was 1 million retailers of equal size it would survey 100 of them and multiply the figures by 10,000 to produce the overall figures. As you correctly point out, there is not a uniform market, and some retailers have to be weighted. But weighting does not include compiling other people's figures and predictions. The methodology ioi gave shows that this is what VG Chartz does. It compiles a mishmash of sales figures, previous trends and guesswork then changes things when the NPD data appears. I've defended this site because I thought it just used a small and therefore unreliable sample size. The truth appears somewhat different 
