There is a new 'Sales Trend Analysis' report (Parts-Reports-Sales Reports-Customer Sales Trend-Report) which highlight's if a customers sales are increasing or decreasing.
This report asks for an Ending Date, reports the month sales for the 12 months up until this date and then a total for the previous 12 months, along with a variance and %.
You can choose to enter a minimum sales value to report as well as selecting all Trade Customers or by Customer Type.