Alright, this was simple after all! I really appreciated all the help you gave me! 
Best regards once again!
:EDIT:
So now, for the slider listbox I have something like this.
Code: ags
I am calling this function on game_start!
On slider change I have something like this:
Code: ags
But when I try to test it (moving the slider) I get this message.
ListBox1SetTopItem: tried to set top to beyond top or bottom of list
I am not sure how to fix it at all.

Best regards once again!
:EDIT:
So now, for the slider listbox I have something like this.
void UpdateSliderListbox()
{
// calculate how many rows of inv items are going to be hidden
int hiddenRows = (ListBox1.ItemCount - ListBox1.RowCount);
if (hiddenRows > 0)
{
SliderListBox1.Max = hiddenRows;
SliderListBox1.Enabled = true;
}
else SliderListBox1.Enabled = false;
}
I am calling this function on game_start!
On slider change I have something like this:
ListBox1.TopItem = (SliderListBox1.Max - SliderListBox1.Value) * ListBox1.ItemCount;
But when I try to test it (moving the slider) I get this message.
ListBox1SetTopItem: tried to set top to beyond top or bottom of list
I am not sure how to fix it at all.