Hello everyone, I have a question about DDD. I'm building a application to learn DDD and I have a question about layering. I have an application that works like this:

UI layer calls => Application Layer -> Domain Layer -> Database

Here is a small example of how the code looks:

//****************UI LAYER************************
//Uses Ioc to get the service from the factory.
//This factory would be in the MyApp.Infrastructure.dll
IImplementationFactory factory = new ImplementationFactory();

//Interface and implementation for Shopping Cart service would be in MyApp.ApplicationLayer.dll
    IShoppingCartService service = factory.GetImplementationFactory<IShoppingCartService>();

    //This is the UI layer,
    //Calling into Application Layer
    //to get the shopping cart for a user.

    //Interface for IShoppingCart would be in MyApp.ApplicationLayer.dll
    //and implementation for IShoppingCart would be in MyApp.Model.
    IShoppingCart shoppingCart = service.GetShoppingCartByUserName(userName);

    //Show shopping cart information.
    //For example, items bought, price, taxes..etc
    ...

    //Pressed Purchase button, so even for when
    //button is pressed.
    //Uses Ioc to get the service from the factory again.
    IImplementationFactory factory = new ImplementationFactory();
    IShoppingCartService service = factory.GetImplementationFactory<IShoppingCartService>();
    service.Purchase(shoppingCart);


    //**********************Application Layer**********************
    public class ShoppingCartService : IShoppingCartService
    {
       public IShoppingCart GetShoppingCartByUserName(string userName)
       {
           //Uses Ioc to get the service from the factory.
           //This factory would be in the MyApp.Infrastructure.dll
           IImplementationFactory factory = new ImplementationFactory();

           //Interface for repository would be in MyApp.Infrastructure.dll
           //but implementation would by in MyApp.Model.dll
           IShoppingCartRepository repository = factory.GetImplementationFactory<IShoppingCartRepository>();

           IShoppingCart shoppingCart = repository.GetShoppingCartByUserName(username);
           //Do shopping cart logic like calculating taxes and stuff
           //I would put these in services but not sure?
           ...

           return shoppingCart;
       }

       public void Purchase(IShoppingCart shoppingCart)
       {
            //Do Purchase logic and calling out to repository
            ...
       }
    }

I've seem to put most of my business rules in services rather than the models and I'm not sure if this is correct? Also, i'm not completely sure if I have the laying correct? Do I have the right pieces in the correct place? Also should my models leave my domain model? In general I'm I doing this correct according DDD?

Thanks!