add_action('paypal_ipn_for_wordpress_payment_status_completed', 'post_to_my_ipn_tbl', 10, 1);

function post_to_my_ipn_tbl($posted) {
    global $wpdb;
    $table_name = $wpdb->prefix . "ipn_data_tbl";
    $first_name = isset($posted['first_name']) ? $posted['first_name'] : '';
    $last_name = isset($posted['last_name']) ? $posted['last_name'] : '';
    $payer_email = isset($posted['payer_email']) ? $posted['payer_email'] : '';
    $wpdb->insert($table_name, array('payer_email' => $payer_email, 'first_name' => $first_name, 'last_name' => $last_name));
}