Ante I think the issue comes from comboBox.SelectedItem, this returns an Object and in your case that Object happens to be a System.Data.DataRowView. I think you'll need to cast the combobox.selectedItem to a value. I'm a VB guy so not sure the syntax for C# but in VB we'd do something like this:
DirectCast(combobox.SelectedItem, DataRowView).Foo
with foo being what ever value you want to pass to the listbox.
Another option that might work, if your intention is to include the value of the combo box in the listbox is to use:
combobox.selectedvalue
this returns and Object but it's actually the object that is being displayed in the combobox weather it be a string, int, etc. Not sure if that helps but I've had to do something very similar to this in the past and that's the solution I came up with.