Как составить запрос на последнего юзера, купившего товар?
Такой запрос делается через оконные функции
SELECT `userID`, `productID`, `date`
  FROM (
    SELECT `userID`, `productID`, `date`,
           ROW_NUMBER() OVER `win` AS `row`
      FROM `table`
      WINDOW `win` AS (PARTITION BY `productID` ORDER BY `date` DESC)
  ) AS `t`
  WHERE `row` = 1
А если так?

SELECT
	Cust.Cust_User,
	Usr.User_Name,
	Cust.Cust_Prod_ID,
	Prod.Prod_Name,
	Cust.Cust_DateTime_Buy
FROM
	Customer as Cust
LEFT JOIN Customer Cust2 ON
	Cust.Cust_User = Cust2.Cust_User
	AND Cust.Cust_Prod_ID = Cust2.Cust_Prod_ID
	AND Cust.Cust_DateTime_Buy < Cust2.Cust_DateTime_Buy
LEFT JOIN Users as Usr ON
	Cust.Cust_User = Usr.User_ID
LEFT JOIN Product as Prod ON
	Cust.Cust_Prod_ID = Prod.Prod_ID
WHERE
	Cust2.Cust_ID IS NULL;