Hi Mohamed,
Have you tried building this out using a summary report yet? Summary reports do allow you to pick how your data is grouped and could be built to show you a summary of data based on that grouping. For example:
Store Rep # of Sales
Store 1 Rep 1 2
Store 1 Rep 1 3
Store 2 Rep 1 2
Store 1 Rep 2 1
Could be come:
Store Rep # of Sales
Store 1 Rep 1 5
Rep 2 1
Store 2 Rep 1 2
You would just want to set the summary report to work off of the fields you want to report against. I hope that suggestion is helpful.