Do a select statement and SUM up the ProductDetails table?
yes..i think of it for displaying total per Products..

And if you sell, how do you how much stock you still have?
coz evertime the user (at my company) entry the data to sold to customer..they have option to select which product to sell..(by its price, invoiceID or OrderID, etc) so it can minus Quantity for the related Product that user select..