Put the list box on a panel. Make the list box tall enough to display all the items at the same time. Make you buttons move the list box up and down so what ever part of the panel you want visible can be seen in the panel. Here's a sample of what I mean using a label. Just create a new winforms app, and paste this over the form1 definition:
public partial class Form1 : Form
{
private System.Windows.Forms.Panel panel1;
private System.Windows.Forms.Label label1;
private System.Windows.Forms.Button button1;
private System.Windows.Forms.Button button2;
public Form1()
{
InitializeComponent();
// Create the controls.
this.panel1 = new System.Windows.Forms.Panel();
this.button1 = new System.Windows.Forms.Button();
this.button2 = new System.Windows.Forms.Button();
this.label1 = new System.Windows.Forms.Label();
//
// panel1
//
this.panel1.Controls.Add(this.label1);
this.panel1.Location = new System.Drawing.Point(93, 12);
this.panel1.Size = new System.Drawing.Size(60, 50);
this.panel1.TabIndex = 0;
//
// button1
//
this.button1.Location = new System.Drawing.Point(12, 12);
this.button1.Size = new System.Drawing.Size(75, 23);
this.button1.TabIndex = 0;
this.button1.Text = "Up";
this.button1.UseVisualStyleBackColor = true;
this.button1.Click += new System.EventHandler(this.scrollUp);
//
// button2
//
this.button2.Location = new System.Drawing.Point(12, 41);
this.button2.Size = new System.Drawing.Size(75, 23);
this.button2.TabIndex = 0;
this.button2.Text = "Down";
this.button2.UseVisualStyleBackColor = true;
this.button2.Click += new System.EventHandler(this.scrollDown);
//
// label1
//
this.label1.Location = new System.Drawing.Point(0, 0);
this.label1.Name = "label1";
this.label1.Size = new System.Drawing.Size(60, 186);
this.label1.TabIndex = 1;
this.label1.Text = "This is a label with some text that goes on for ages and ages to " +
"demonstrate how it is possible to manually scroll a control using a panel.";
