Have you considered to just show two columns? One with the values (contact, car no., salesperson, price and a host of other details), the second done with the customer name and values, but place a drop down to select each customer. This approach would preserve the vertical list of features so that you only need to select customers.
Adding arrows would allow you to quickly scan each customer as their data is shown below.
What if table columns cannot fit on webpage?
If, as you mentioned, the primary user need and flow is to display data for the number of cars sold that month, you could start with a table with the last 12 (or so) months and the total of cars sold for each month, then (upon month selection) drill down to the next interface titled by month (back/forward arrows on either side) and display detail by the next primary criteria, whether customer's name or else. A quick research session with users will help define the user flow and prioritize user needs.