In cases like this, I usually make a table with all values I want displayed and do a left join to my data set. So in your case, I would make a table called "months" with a single date column that stores the first day of the month or maybe two columns with the first and last day and do join like this:
SELECT m.month_first, COUNT(x.date_of_car_sale) as sales
FROM months m
LEFT JOIN (
SELECT date_of_car_sale
FROM sales
) x ON m.month_first <= x.date_of_car_sale
AND m.month_last >= x.date_of_car_sale
GROUP BY m.month_first
ORDER BY m.month_first