The average household income in the country might have risen significantly over the past three years. But this gives just the average value.
This is a good point. Although, you can state it in a different way... That is, the average household income can also rise if the rich earn more than they did in the past. In that case, the average may not indicate the purchasing power of the actual consumers of the megamart.
The other points are pertinent. An additional point maybe the fact that, the trend has taken place over a very short period i.e. three years.
You are definitely getting better with each essay. Good luck with your GRE. I'm also thinking about taking it this August.
P.S. Why don't you post the GRE (Analysis of an issue) essay?