Trying to create a Data Model It may sound stupid but it's my first experience ever with databases.

It's a simple database for a car accessories store. I was hoping when making a sale on a certain item it effects the quantity of this item, not sure if this is possible with this design. Also as shown in the image an item could be for one car or more so I thought it should be many to many.
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).