MYSQL joins does not work as expected

I am trying to create a join statement to solve my problem but cannot get my head around it.

I am a new to join statements so please bear with me if my sql statement is nonsense.

I have two tables, one is a table of questions where users have asked questions about items for sale.

Second is a table of items that the user has asked a question about.

Table one called questions consists of question_ref, questioner_user_ref, item_ref, seller_ref, question_text, timestamp

questions
===========
+--------------+--------------------+---------+-----------+--------------+----------+
| question_ref |questioner_user_ref |item_ref |seller_ref |question_text |timestamp |
+--------------+--------------------+---------+-----------+--------------+----------+

Table two called my_item_comments consists of questioner_ref, item_ref, last_question_ref

my_item_comments
===========
+---------------+---------+------------------+
|questioner_ref |item_ref |last_question_ref |
+---------------+---------+------------------+

I have set up table two to keep track of items that the user has asked questions about so they can be informed when someone else asks the seller a question or the seller answers.

So I want to create a recordset of questions that a). Someone has answered a question about an item the user is selling b). A seller has replied to a question the user asked,
c). A third user has asked a question about an item that the user has also asked a question about.

A bit like facebook's commenting system, where you are informed about comments people have made on statuses that you have commented on.

So my current sql statement is as follows

$user_ref= current logged in user

  $sql=mysql_query("
    SELECT * FROM questions
    LEFT JOIN my_item_comments
    ON questions.item_ref=my_item_comments.item_ref
    WHERE questions.questioner_user_ref!='$user_ref'
    AND (questions.seller_ref='$user_ref' OR questions.item_ref=my_item_comments.item_ref
    ORDER BY timestamp DESC");

The results don't work and I think its because of the OR questions.item_ref=my_item_comments.item_ref but for the life of me I cannot work it out.

Any help would be greatly appreciated, even if it means restructuring my database with more tables or new fields in the tables.

thanks in advance, Barry

---

SELECT * FROM questions
LEFT JOIN my_item_comments ON questions.item_ref=my_item_comments.item_ref
WHERE
   questions.questioner_user_ref != '$user_ref' AND
   (questions.seller_ref='$user_ref' OR questions.item_ref=my_item_comments.item_ref)
ORDER BY timestamp DESC

I think you were missing a ) to enclose the OR clause (in the example above I added it)