Trying to create a Data Model
You need to read a business data modeling book such as those by Silverston, Hay, or Fowler
You are confusing a Product, which is the specification of an Asset (such as a vehicle). A product does not have a quantity. For cars, the Product has a model number (Mazda 3) and the Asset has the serial number (VIN xxx).
You should also read up on Table Inheritance (Fowler).