Hi Jason,
With transformations, you only have certain data available to you. While you would have product data (assuming you are working a Page Type that is a product), getting the sales of that product would be a pretty abstract concept, requiring a few API calls to calculate. For this reason, you would probably want to make a custom macro to get the values, then call that macro within your transformation.
You can find out more about custom macros here:
https://docs.kentico.com/display/K9/Registering+custom+macro+methods
I also recently published a blog on creating a custom macro (in this case a Loren Ipsum generator), however, the steps would be the same for you.
http://devnet.kentico.com/articles/creating-a-lorem-ipsum-generator-macro-in-kentico
Transformations do have a number of built-in functions you can call. You can find a list of them here:
https://docs.kentico.com/display/K9/Reference+-+Transformation+methods
Lastly, you may be able to get some inspiration from the Ecommerce reports, specifically the Reporting/E-commerce/Products/Top product by number of purchases report. This report uses SQL calls to calucalte the number of times a product has been bought. You could use this SQL as a guide if you wanted to have your own SQL call in your custom macro.
/* Variable determines whether global products should be displayed. */
DECLARE @GlobalProducts bit;
SET @GlobalProducts = {%Settings.CMSStoreAllowGlobalProducts?1:0|(identity)GlobalAdministrator%};
/* Selects the product meeting the conditions from the filter and
the number of orders in which it occures */
SELECT TOP (@Limit)
ISNULL(parent.SKUName + ': ', '') + product.SKUName + ISNULL(' (' + COM_OptionCategory.CategoryDisplayName + ')', '') AS '{$product_list.productname$}',
product.SKUNumber AS '{$product_list.productnumber$}',
ISNULL(Departments.DepartmentDisplayName, parentDepartment.DepartmentDisplayName) AS '{$objecttype.com_department$}',
ISNULL(Manufacturers.ManufacturerDisplayName,parentManufacturer.ManufacturerDisplayName) AS '{$objecttype.com_manufacturer$}',
ISNULL(Suppliers.SupplierDisplayName,parentSupplier.SupplierDisplayName) AS '{$objecttype.com_supplier$}',
CAST(product.SKUPrice AS DECIMAL(38,2)) AS '{$com.inventory.price$}',
ISNULL(COUNT(DISTINCT OrderID),0) AS '{$com.purchasedproductsreport.numberofpurchases$}'
FROM
COM_SKU product
LEFT JOIN COM_OrderItem
ON product.SKUID = OrderItemSKUID
LEFT JOIN COM_Order
ON OrderID = OrderItemOrderID
AND (ISNULL(OrderIsPaid,0) = 1 OR ISNULL(@OrderIsPaid,0) = 0)
AND (OrderDate >= @FromDate OR @FromDate IS NULL)
AND (OrderDate <= DATEADD(day,1,@ToDate) OR @ToDate IS NULL)
AND (product.SKUOptionCategoryID IS NULL OR (((@GlobalOptions = 1 AND product.SKUSiteID IS NULL) OR product.SKUSiteID = @CMSContextCurrentSiteID)))
AND (product.SKUOptionCategoryID IS NOT NULL OR ((@GlobalProducts = 1 AND product.SKUSiteID IS NULL) OR product.SKUSiteID = @CMSContextCurrentSiteID))
LEFT JOIN COM_Department AS Departments
ON Departments.DepartmentID = product.SKUDepartmentID
LEFT JOIN COM_Manufacturer AS Manufacturers
ON product.SKUManufacturerID = Manufacturers.ManufacturerID
LEFT JOIN COM_Supplier AS Suppliers
ON Suppliers.SupplierID = product.SKUSupplierID
LEFT JOIN COM_OptionCategory
ON CategoryID = product.SKUOptionCategoryID
LEFT JOIN COM_SKU parent
ON product.SKUParentSKUID = parent.SKUID
LEFT JOIN COM_Department parentDepartment
ON parentDepartment.DepartmentID = parent.SKUDepartmentID
LEFT JOIN COM_Manufacturer parentManufacturer
ON parent.SKUManufacturerID = parentManufacturer.ManufacturerID
LEFT JOIN COM_Supplier parentSupplier
ON parentSupplier.SupplierID = parent.SKUSupplierID
WHERE
(product.SKUOptionCategoryID IS NULL OR ((@GlobalOptions = 1 AND product.SKUSiteID IS NULL) OR product.SKUSiteID = @CMSContextCurrentSiteID))
AND (product.SKUOptionCategoryID IS NOT NULL OR ((@GlobalProducts = 1 AND product.SKUSiteID IS NULL) OR product.SKUSiteID = @CMSContextCurrentSiteID))
AND (product.SKUOptionCategoryID IS NULL OR (CategoryType = 'PRODUCTS'))
AND ((product.SKUEnabled = 1 AND (parent.SKUEnabled IS NULL OR parent.SKUEnabled = 1))
OR (product.SKUID IN (SELECT OrderItemSKUID FROM COM_OrderItem WHERE OrderItemOrderID IN (SELECT OrderID FROM COM_Order WHERE OrderSiteID=@CMSContextCurrentSiteID))))
AND (@DepartmentID = product.SKUDepartmentID OR @DepartmentID < 0 OR (@DepartmentID IS NULL AND product.SKUDepartmentID IS NULL))
AND (@ManufacturerID = product.SKUManufacturerID OR @ManufacturerID < 0 OR (@ManufacturerID IS NULL AND product.SKUManufacturerID IS NULL))
AND (@SupplierID = product.SKUSupplierID OR @SupplierID < 0 OR (@SupplierID IS NULL AND product.SKUSupplierID IS NULL))
AND (@PublicStatusID = product.SKUPublicStatusID OR @PublicStatusID < 0 OR (@PublicStatusID IS NULL AND product.SKUPublicStatusID IS NULL))
AND (@InternalStatusID = product.SKUInternalStatusID OR @InternalStatusID < 0 OR (@InternalStatusID IS NULL AND product.SKUInternalStatusID IS NULL))
AND (product.SKUName LIKE ('%'+@ProductName+'%') OR @ProductName IS NULL)
AND (product.SKUNumber LIKE ('%'+@ProductNumber+'%') OR @ProductNumber IS NULL OR @ProductNumber = '')
AND COM_Order.OrderSiteID = @CMSContextCurrentSiteID
GROUP BY
product.SKUID,
ISNULL(parent.SKUName + ': ', '') + product.SKUName + ISNULL(' (' + COM_OptionCategory.CategoryDisplayName + ')', ''),
product.SKUNumber,
product.SKUPrice,
ISNULL(Departments.DepartmentDisplayName, parentDepartment.DepartmentDisplayName),
ISNULL(Manufacturers.ManufacturerDisplayName,parentManufacturer.ManufacturerDisplayName),
ISNULL(Suppliers.SupplierDisplayName,parentSupplier.SupplierDisplayName)
HAVING
ISNULL(COUNT(DISTINCT OrderID),0) > 0
ORDER BY
COUNT(DISTINCT OrderID) DESC,
ISNULL(parent.SKUName + ': ', '') + product.SKUName + ISNULL(' (' + COM_OptionCategory.CategoryDisplayName + ')', '') ASC,
product.SKUNumber ASC