private void button1_Click(object sender, EventArgs e)
        {
            UserRentals userRentals;

            this.Visible = false;
            string selectedUser = comboBox1.SelectedItem.ToString();
            string type         = mrd.getUserType(selectedUser);

            if (type == "customer")
            {
                userRentals = new UserRentals(selectedUser);
                userRentals.Show();
            }

            if (type == "seller")
            {
                SellerMenu sellerMenu = new SellerMenu();
                sellerMenu.Show();
            }
        }