List<> duplicate object problem

thank you very much.. i just read your posts.. and thinking to try first one.. hope everything will be ok.. i am not familiar with  T stuff enought to work with it.. so i hope both solution has same funcionality

Another option is that you need to provide a object deep copy functionality, which will copy the contents of an object to another object. Ref won't copy.
Step1
Create below extension method in you Form or in side  any utility class
public static class ExtensionMethods
    {
        // Deep clone
        public static T DeepClone(this T a)
        {
            using (MemoryStream stream = new MemoryStream())
            {
                BinaryFormatter formatter = new BinaryFormatter();
                formatter.Serialize(stream, a);
                stream.Position = 0;
                return (T)formatter.Deserialize(stream);
            }
        }
    }
Below NameSpaces are mandatory
using System.IO;
using System.Runtime.Serialization.Formatters.Binary;
Step 2
Make your Article class serializable by adding just ne attribute, as then only it supports deep clone
[Serializable]
    public class Article
...............
..................
Step 3
Now you can modify your button click like below which is creatng a deep copy of selected ListItem and you will automatically get fresh object each time.
 private void button1_Click(object sender, EventArgs e)
        {
            Article artForSell = ((Article)listBox1.SelectedItem).DeepClone

();
            artForSell.Amount = double.Parse(textBox1.Text);
            ArtsOnAccount.Add(artForSell);
            if (ArtsOnAccountChange != null)
            {
                ArtsOnAccountChange(sender, e);
            }           
        }

Hi,
Here problem is that each time you are using same Ref. object. But you really need to create new object each time rather than using "SelectedItem" of ListBox. Selected item always same only. So correct it like below by create new object.
private void button1_Click(object sender, EventArgs e)
        {
            Article artForSell = (Article)listBox1.SelectedItem;
            Article newArtForSell = new Article { Key = artForSell.Key, Name = artForSell.Name, Price = artForSell.Price, Stock = artForSell.Stock };
            newArtForSell.Amount = double.Parse(textBox1.Text);
            ArtsOnAccount.Add(newArtForSell);
            if (ArtsOnAccountChange != null)
            {
                ArtsOnAccountChange(sender, e);
            }           
        }

Here is the project
to see the bug..
run the exe, or build the project, select Milk in ListBox1, than insert 1 for amount, and than press ADD button, 
than again, select Milk in ListBox1, for amount insert for example 3, and than press ADD button.. 

In the ListBox2 shoud be 2 Items:

Milk 1 * 5

Milk 3 * 5

but instead of that, after adding the second Milk,  ListBox2 Contains:

Milk 3*5

Milk 3*5
Allso in Debug mode if you "Watch"ArtsOnAccount" variable, same thing happens there.. so it is List problem.. something i don't know about lists.. 

Hope you see the code and help me solve this.. thanks

This sounds like a typical shopping cart application.  The question I'd have is how are you finding the article to decrement.  You say you are doing a "-=" to set the new value, but how did you get the article to do this.  Can you share your code that does the decrementing?