There are plenty of ways already available to you.
Posting in the EE thread which you should be doing anyway so you can use the feedback system, will allow you to use the search engine to locate the thread.
Subscribe to the thread and use the subscription system.
Bookmark the thread in your browser.
How did you make your initial contact with the seller?
-IM: send & save a copy which you should be doing, copies of all communications are crucial to dispute resolution. You'll have the screen name and can search the EE for threads started by.
-Site generated email: send & save a copy which you should be doing, copies of all communications are crucial to any dispute resolution. You'll have the screen name and can search the EE for threads started by.
-Email listed in the ad thread: You said that you have the email address, the web based email accounts I use are highly searchable.
Registered emails aren't searchable. Email posted in an ad is searchable.
Searching for the item is can be done using the site search engine or Bing and Google using site:ar15.com to specify the board.