It's not that difficult. Here's my concept:
1. Declaration of ListBox.
<ListBox SelectionMode="Multiple" x:Name="xList"/>
The important thing is setting SelectionMode.
2. Then creating and saving the string on ButtonClick. Nothing sophisticated here just:
string s="";
foreach(System.Windows.Controls.ListBoxItem li in xList.SelectedItems)
{
s+= (li.Content as (whatever object youre binding here)).(Field you need to save);
s+=";";
}
SaveToDB(s);