Wordpress - WPDB Insert or if exists Update

Have you tried $wpdb->replace. According to WP Codex:

Replace a row in a table if it exists or insert a new row in a table if the row did not already exist.

I have tried myself in some plugins and it does the work when trying to avoid unique IDs duplication errors, etc.

More info in the codex

First, you are using prepare incorrectly. You seem to have $wpdb->update's arguments wrapped in $wpdb->prepare like that. That won't work. In effect, you are passing update a single argument-- the output of prepare. Try something simple like the following and you will see why that won't work:

$post_id = 123;
$item_stock = 567;
var_dump(
  $wpdb->prepare(
    $wpdb->prefix.'item_info',
    array(
        'post_id'       => $post_id,
        'item_stock'    => $item_stock
    ),
    array('post_id' => $post_id)
  )
);

And $wpdb->update() runs prepare for you.

Second, if this were me, I skip the helper function bloat and write a proper ON DUPLICATE KEY UPDATE query:

$sql = "INSERT INTO {$wpdb->prefix}item_info (post_id,item_stock) VALUES (%d,%s) ON DUPLICATE KEY UPDATE item_stock = %s";
// var_dump($sql); // debug
$sql = $wpdb->prepare($sql,$post_id,$item_stock,$item_stock);
// var_dump($sql); // debug
$wpdb->query($sql);

This assumes that post_id is a UNIQUE index or PRIMARY KEY. If your table structure is what I think it is, let the database handle it.