It is necessary to process personal information for that purpose. That's what the sales records are.
> top 5 customers
You probably have to declare that the data is processed for that purpose in general terms but I don't see why consent would be necessary. Anyway, this analytics service claims it doesn't do this kind of analysis.
Obviously it depends on the system involved, but there should be no need to touch any column containing personally identifying information in order to calculate aggregate sales statistics for each of your products.
