property nextSelectedRow

This property returns the index of the next selected row. Note that you should call firstSelectedRow prior to calling this property.

var list = scene.getBoxById("myList");
if (list)
{
    var sel = list.selection;
    if (sel)
    {
        var selRow = sel.firstSelectedRow;
        while (selRow != -1)
        {
            Foo(selRow); // process row
            selRow = sel.nextSelectedRow;
        }
    }
}