If you have the user list plugin installed, you can show all listings by any user and then you can subscribe to that results list using the subscribe tool on the search sidebar. Every time a user ads a new listing, it will be added to your "manage your alerts" lists. Check it out. It works.
So, I guess the feature you want is already there...if you use the following plugin. You could always add another link on the list results to "subscribe to the seller"...and use the same code that's used in the search sidebar form for "subscribe to this seller".
http://sourceforge.net/projects/osclass/files/Plugins/list-seller-items/Also, I've tested, and the search does get results for both title and description fields for me. You can see the search term highlighted (bold) in the results list. Common words like the, or, for, etc will not show search results for me, which is fine.